temp gauge

I have a 93 4.3L 5 speed. s-10, I just changed the water pump because it was leaking through the weep hole. The problem I have now is that the temp gauge keep going up and down constantly, I changed the thermostat twice because I thought the first one was defective, I changed the temp. sending unit and it still keeps going up and down. Checked all the wiring and nothing seems to be wrong. the manaul talks about a reverse circulating water pump, but the guy at autozone told me he gave me the correct one required for the truck. He said a reverse circulating pump would require more belts than just a serpentine belt. The guy from checker told me the same thing. If its the gauge itself, how do I check it. I replaced the instrument cluster about 6 months ago because my fuel gauge would not work so pulling it apart would not be a problem. anybody have any ideas. Also I took the thermostat off for a day and noticed that the gauge does not go up and down what would the harm be of running without a thermostat.

You actually have 2 temp sending units... 1 for the computer and 1 for the gauge... The 1 for the gauge is on the drivers side block, the one for the ecm is near the thermostat housing. That said, you can put a reverse flow water pump on without additional belts.... Possible they could have given you the wrong pump. Also you may have a clog or you may simply have air in the system. Did you bleed the coolant system properly?