Mariva 1.4. Why does temperature gauge reading fluctuate?

After the MIL is turned off or the battery was disconnected for some time, all monitors are set to incomplete. However with several short trips with engine getting at least to hot enough for thermostat to open, (I believe 8 is the magic number of trips) most monitors are set, you may be able to pass the test with only one incomplete only.

I would check the thermostat first. Remove the thermostat and place it in a pan of water on the stove. Place a thermometer in the water with the thermostat and slowly increase the temperature until you have reached 194 degrees Fahrenheit or 90 degrees Celsius. Watch closely for the thermostat to open slightly. If it opens then the thermostat is good. If it does not open, replace it. Try that first and see what you come up with.

Do you have an OBDII scanner to check the real temperature? Want to make sure it's not overheating. If it's normal, could be your temperature sensor starting to go out, or ignition switch. Any check engine lights? If so, get the code for me, can go from there. At a constant speed, you sure it wasn't a misfire instead? Check your trans fluid, make sure it's full. I was just in Tahoe, pretty sad not much snow at all.

This is totally normal. All radiators contain an antifreeze/ antiboil component when filled as per manufacturers instructions. This stabilizers the temp of the coolant, reducing fluctuations in temperature. The thermostat is heat responsive. Basically a piece of spring steel heated to a temperature then cooled, so as to retain the memory of when to react. Replacing the water temp sensor on this occasion was probably unnecessary, tho in saying that, I have replaced mine & friends with beneficial results.

A thermostat should only allow flow after reaching its desired temp, 195, however, if this is just idling and you have a brand new aluminum 4 core radiator then it may really not be reaching temp and reading low because of no load and a great radiator. I assume top of the intake manifold is where your temp sender is? Did you use a laser infra red temp reader and point it at the block in several spots? You may see175, 185, etc all over the side of the eng. The neck of the thermostat housing should be the closest to 195 but the lower hose might read 140 / 150. Alum radiators work awesome so when that thermostat does open it's a rush of cooler coolant. What is it doing when you drive steadily @ 40 - 50 mph? If the gauge reads say 140 and just fluctuates slightly from that then I would say yes, replace the thermostat. If your gauge shows it now reaching temperature then it's just that your radiator is doing a helluva job when it idles. BTW, electric fans? On a switch or temp sender? Or mechanical fan always pulling air even at idle?
Hope this helps

Have the system flushed again and replace thew thermostat.

Here is the OBD2 emission system drive cycle test procedure.
Engine coolant temp must be below 70c (158f) before starting the engine for either the first or second trip
The fuel level should be less than 1/2 full
First trip:
Start engine, idle for at least 1.5 minutes
Drive 3 minutes, hold b/f schdl: 1.5-2.0 msec, selector lever "3rd" eng rpm 1,800-2,000 A/C switch:ON Hold the recommended b/f schdl range. Allow road speed to vary as necessary.
Drive at 55-65 mph for 1.5 min b/f schdl 2.0-3.2 msec, selector lever "5th" Engine rpm 2,200-3,000
IGN "off" for at least 10 seconds (not more than 5 min)
Dive at 50-60 mph for 3 minutes Selector lever "4th" keep engine speed above 3,000 rpm Allow speed to vary if necessary. Do not decelerate for more than 3 consecutive seconds. A/C switch ON.
Drive at a steady state cruise of 53-58mhp b/f schdl more than 1.8 msec selector lever "5th" eng rpm 2,400-2,600
Downshift to "4th" and decel more than 5 seconds without breaking, then idle for 1 minute.
Drive two minutes b/f schdl less than 2.0 msec Accelerate to 41 mph, decelerate to 34 mph, accelerate to 41mph. Do not completely release the accelerator. A/C switch off.
Idle 1 minute in park or neutral
Drive two minutes Steady state cruise at 31-44 mph. Selector lever "4th" A/C switch OFF. Hold the accelerator pedal as steady as possible. Allow speed to change if necessary.
End of first trip. Turn the key off the begin the second trip.
Second trip:
Start engine, idle for at least 1.5 minutes
Drive 3 minutes, hold b/f schdl: 1.5-2.0 msec, selector lever "3rd" eng rpm 1,800-2,000 A/C switch:ON Hold the recommended b/f schdl range. Allow road speed to vary as necessary.
Drive at 55-65 mph for 1.5 min b/f schdl 2.0-3.2 msec, selector lever "5th" Engine rpm 2,200-3,000
IGN "off" add four gallons of fuel
Dive at 50-60 mph for 3 minutes Selector lever "4th" keep engine speed above 3,000 rpm Allow speed to vary if necessary. Do not decelerate for more than 3 consecutive seconds. A/C switch ON.
Drive at a steady state cruise of 53-58mhp b/f schdl more than 1.8 msec selector lever "5th" eng rpm 2,400-2,600
Downshift to "4th" and decel more than 5 seconds without breaking, then idle for 1 minute.
Drive two minutes b/f schdl less than 2.0 msec Accelerate to 41 mph, decelerate to 34 mph, accelerate to 41mph. Do not completely release the accelerator. A/C switch off.
Idle 1 minute in park or neutral
Drive two minutes Steady state cruise at 31-44 mph. Selector lever "4th" A/C switch OFF. Hold the accelerator pedal as steady as possible. Allow speed to change if necessary. I especially like the part where they want you to add four gallons of fuel. Not three gallons, not five gallons, four gallons. Gotta love those Nissan engineers. ...and of course they state that if the trace is not followed exactly (i.e. you botch the acceleration/deceleration) you have to start all over again from scratch

The following drive cycle can be used as a guide to assist with re-setting monitors when a vehicle specific drive cycle cannot be located. This OBDII drive cycle begins with a cold start (coolant temperature below 122 degrees F and the coolant and air temperature sensors within 11 degrees of one another). This condition can be achieved by allowing the vehicle to "sit" overnight, and then by beginning the drive cycle the next day. Most drive cycles will be difficult to follow exactly under normal driving conditions, so the driver should exercise caution, road safety, and courtesy to others.

1. . Start the engine. Idle the engine in drive for two and a half minutes with the A/C and rear defroster on.
2. Turn the A/C and rear defrost off, and accelerate to 55 mph at half throttle.
3. Hold at a steady speed of 55 mph for three minutes.
4. Decelerate (coast down) to 20 mph without braking or depressing the clutch.
5. Accelerate back to 55 to 60 mph at ¾ throttle.
6. Hold at a steady speed of 55 to 60 mph for five minutes.
7. Decelerate (coast down) to a stop without braking.

check coolant level ,must be low ,chck for coolant loss

I would check the throttle position sensor