My heat in my truck comes and goes but the temp gauage stays at normal range

It is possible that the coolant is a little low causing the heater core to get "air locked" Check your coolant level. There could also be a problem with your water pump not able to pump correctly due to a worn or corroded impeller. This will usually cause the heater temperature to fluctuate widely with engine RPM. There is allways some fluctuation caused by RPM, but if it is a large fluctuation, it usually means low water pump output or possibly a partially clogged heater core.
Feel the hoses going into and out of the heater core. They should both be hot. There may be a 20 or 30 degree difference in temperature, but both should feel very hot to the touch. If one is really hot and the other just feels warm, the heater core is probably clogged. If you have access to an infrared thermometer, taking the temperature of the hoses is the best way to check this.
If both hoses feel "Warm" at idle and get really hot when the throttle is cracked open for a short period (20-30 Seconds ), the water pump may be the problem.

If the temp gauge is normal then the most common reason may be a stuck open thermostat, now if it takes a while to get heat then the thermostat may be stuck open a bit, i would start there and after replacing the thermostat i would remove the hoses from the heatercore and use a water hose to flush the heater core, make sure to go both ways flushing the core, the heater core gets plugged sometimes, i have seen this alot, the last and the first thing to do is make sure the coolant level is full, low coolant is the number one reason for this type of problem. good luck and i hope this is helpful.

Sounds like your heater core is beginning to clog
hope this helps
its under the passenger side dash
leave feedback if you can thanks
good luck