91 lumina euro keeps overheating chevalltheway

1991 lumina 3.1 euro v6 CAR

hey my lumina's engine was rebulit 5 months ago and with it i had the guy replace the water pump and the temp senser on the block. once the engine was replaced it was all cool until the last 2 weeks! about a month ago my cars temp senser went(as in moved from side to side and then staying at full heat) and i have been putting it off to be fixed because it was more for looks then anything. 2 weeks ago my car started dieing in the middle of the road but only on acceleration beetween 3-5000 RPM and then it would start again in no more then 30 sec(pedaling made no diffrence).

problem got worse to the point that it would stall every time i drove it, but then it seemed to get better as in the car would just jump around a little and keep going never stalling like it had been. but dispite it getting better i would still stall from time to time and today i replaced the temp switch in the engine bay thinking that, that could be the reason it was stalling, the temper gage works now but it is reading 125C (meaning full heat that the temp can read)(the temp of the engine will go to the half way mark on the gage which is right but only on the highway) and even with that temp the fans almost never turn on(the A/C fan has never turned on)(and before i got the temp gage fixed today the other fan came on everytime the car got hot). replaced the fuel filter thinking that could also be the reason why it is dieing for no reason, put STP fuel injector cleaner and cheaked for bad gas, just also replaced the thermastat.Also just got a electrical test done on the car and no numbers came up for the stalling issue, new fluid and no leaks that i can see........SOMEONE HELP!!!!