Temp guage fluctuates down to bottom when 'theft system' light comes on (flashes every second or two) and then goes back up when light goes out. 'Check guages' light flashes on right after theft system light comes on also and then goes out when theft system light goes out. Is this possible a ground problem somewhere? Where do I start with this one?

I suggest changing the cooling thermostat to make sure it's not getting stuck causing temporary overheating and flush the system and put fresh coolant in.
The guage will go hotter then go back to normal every time the thermostat opens to let cooled water into the engine block which is normal. Time for concern is when the guage goes hot and stays there along with bubbling sounds and coolant overflow onto the ground. Which you probably already know.

Fan clutch most likely went bad

They make a connector for the sender unit and they have to usually be replaced. Watch which way it goes on since they are a little cumbersome plugging back on...I prefer soldering the new connectors on too.

The Northstar engines were notorious for head gasket problems. You likely put a band aid on it when surgery is required. I'd recommend that you have the cooling system flushed & pressure tested as a start.

This is most likely 2 problems. The ABS/Brake light and speedo issue will be the speed sensor located in the read differential housing. The temp guage issue (and I'm assuming this is at idle) could be thermostat or temp sending unit. The fluctuation is causes by the electric cooling fan cycling on and off to control the temp. However, most of the time this fluctuation should be from about 195 - 220 degrees.

temperature sender unit top of engine cylinder head make sure its okay
and make sure wire going to it is not grounding
if it grounds or earths it will send the gauge up and down

Usually one of the head gaskets are bad normally the drivers side from what I have seen, typical problem, i have been down that road also

The sending unit for the oil guage will cause the oil guage to fluctuate this is caused from some sludge build up that is more than likely stuck in the sending unit tube. Use Gunk motor flush on your next oil change this should help the oil guage problem.( make sure you read the direction on the back of the bottle)
As for the check engine light you can take it to Auto Zone and have them scan the computer. Sometimes the EGR valve will build up with carbon causing the valve to stick slightly open which will make the idle on the car fluctuate up and down. Its also possible you need O2 sensors.

i had the same prob. w my 97 grand am. i bought a new ignition switch and installed it...after installing you have to reset the theft system. all you have to do to reset is turn the key far enough for it to bump the motor over then leave the key in the run position for about ten to twenty minutes...might hook bat. to charger cause if it dies in process you have to start all over...the theft light will either blink then stay on or go out completly....