There is a temp sensor that the ECU uses, next to the temp gauge sender, in the upper pipe (in front of the inlet manifold, next to the timing cover)
This allows the ECU to adjust the mixture based on coolant temp.
I changed mine about 4 months before selling the car for the same issue, and this fixed it.
Looks like this, but may have a red, black, or blue top.
Number 1 in this image: