So, I had this problem, every time my RPM:s went over 2.5 - 3.0k my temps started to climb up like hell, was wery annoying to can't drive past 120kph or it redlined...
In the city it stayed about 230*F, (it fluctuated a little..

Then (finally) I ripped the thermostat out and felt that the spring was a little ..ehh.. I don't know the word..non-stiff..?? well I ripped the center part of it out, and put all back together, and WOW!!
at highways temperature stayed at 160*F (if less) and at the city it was around 220*F
I know that its a little low temp at highway, but this is only temporary solution, at monday i'm going for 170* thermostat!!!

PS. I was this !------! near to sell this car! (joke.)

PSS. Every one said that it couldn't be the t-stat only..

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'89 Formula 305 TPI T-5
Check out my car HERE!