the temp gauge on my uncles truck goes up to 195 then drops to 165 then back to 195 back and forth we changed the thermostat and i am sure that there are no leaks the system is full of coolant no air in the system its a 95 chevy 1500 with 5.7 the truck runs fine no cel if the temp sensor was bad wouldnt the check engine light come on

 

There may be 2 engine temp senors on that engine. One for the computer ECT (beside thermostat) and one for the dash gauge (side of block).

 

If it makes the changes slowly I'd say the gauge is working properly as well as the stat. If it changes quickly I'd look at electrical connections.

 

While your temp gauge isnt the most accurate, it's the relative readings and repeatability of the symptoms that count. There's a couple reasons for this to happen:

Air bubble.
The scariest scenario is that it's a symptom of a small crack in a head gasket. Despite conventional analysis, it's not the loss of coolant into combustion chamber that causes it, it's the combustion gases {185 psi -or much more- v 15 psi} into cooling that creates it.
Can be found by testing for combustion gases in coolant reservoir using gas sniffer. Or by looking for bubbles from barb fitting and clear tubing into sender hole, while cranking engine.
A hint is there's often a little sludge on the underside of the radiator cap with otherwise clear coolant.
Another hint. No heat from heater while warming then all of a sudden full heat available.

The other way it can happen is the waterpump seal isnt perfect and it pulls in air as the engine cools. Unlikely if you have a good cap and cap recovery valve doesnt stick. There often is some weep from the underside of bearing housing.

But the most reasonable cause is a crap thermostat. Even new, out of the box. Think about it.. you can buy a new one for about 8 bucks... putting the cost of mfr at less than $1.98.

A sticky thermostat could cause your symptoms. Usually on warmup, gauge will overshoot the 195 point at first, then settle to a lesser fluctuation while engine is warm. Unfortunately this also is common with bad head gasket.

Several of us here had thermostat problems last fall/winter, where the engine was barely reaching 165 deg, even though they'd worked properly before. When I took mine out, I found it was warped and never completely closed.
Buy higher end thermostats and make sure it has a bleed hole or channel that allows air to get past the plate. In most this is a small hole in the plate with a little dongle bead in it that obstructs the hole with coolant flow pressure.

Another remote possibility is a bad sensor where the expansion of the element varies the designed resistance more than it should. Same would apply to the gauge itself. Like I say possible but unlikely.
How to determine:
ECT doesnt follow what the gauge shows.
If you have OBDII then watch ECT with a scan tool of course.
In a pre-OBDII, you can find this by temporarily splicing in twisted pair wires to the ECT circuit and reading the voltage while driving and watching DVM voltage readout. If it doesnt vary much and gauge does, that shows the gauge circuit is wrong.