My 328i 97, 4 door, manual, 120k, runs very poorly when the weather outside is above 80 or so. No problems at all when the temperature outside is cold or cool. The check engine light comes on and then goes off in the weather is cool for a couple of days.
The car is not overheating and the radiator is full. It is becoming a bigger problem due to summer.

It lacks power, sounds rough and feels rough, feels like a misfire.

Replace fuel filter and clean your injectors. The better performance you feel due to colder weather is probably nothing but a placebo.

Replace fuel filter and clean your injectors. The better performance you feel due to colder weather is probably nothing but a placebo.

cooler weather will make a huge diff

My 328i 97, 4 door, manual, 120k, runs very poorly when the weather outside is above 80 or so. No problems at all when the temperature outside is cold or cool. The check engine light comes on and then goes off in the weather is cool for a couple of days.
The car is not overheating and the radiator is full. It is becoming a bigger problem due to summer.

It lacks power, sounds rough and feels rough, feels like a misfire.

Worse in warmer weather? Hmmm....those symptoms sounds like it's running lean. Vapor lock in the fuel system? How's your fuel pumps and tank pressure?

Step one would be to read the DME fault memory.

I have had to it to autozone to get the codes pulled.

Lots of misfires, lots of low compression, and several emissions codes.