I recently replaced a stock water pump with a hi flow milodon unit.  When I did that I swapped out the thermostat with a hi flow milodon unit.  The problem that I am seeing is that the temp gauge hits the red before dropping into the normal range.  I have replaced the thermostat two times.  The second unit was Milodon and the third was JET.  Any ideas?  

What year is the car ?  Have you used another gauge to verify the problem ?  Does the temp rise slow or fast?  Does it cool off at the same rate ?  Did it do this before, after or both ? Just some questions I have to try to help. The problem does sound like a sticking thermostat but what are the chances of having three bad ones.  If the car is a 94-95, they were bad about holding air in the upper radiator hose aver you broke into the cooling system because the water neck is at the same height as the radiator cap.  I used to have to jack up the front of my car to bleed the system. Just an idea but I need more info.  

It is a 1990 LX with a central coast long block (5.0 -w- gt-40P heads, b303).  It wasn't doing it before the swap.  I thought that it was a sticking thermostat, because it heats up to the red (270 deg.)hovers there for a couple of minutes and then drops to normal operating temp.  It does this every ten minutes or so.  Didn't think to try another gauge as it didn't do it before the swap.  I will try to jack it just in case.
Thanks  

P.S.  When referring to the swap I mean the water pump/thermostat.  

Your problem sounds so much like a bad thermostat or air in the system.  Try disconnecting the upper radiator hose from the radiator and filling the hose up all the way. Repeat until no air bubbles rise from the engine. Then quickly reatttach it to the radiator.  Maybe try running it without the thermostat installed.  Or try running it with both hose clamps loose to let it bleed itself out. Just some more ideas.