[dtw]

Is it stock?

Is the temp accurate (per Jamie above)?

If yes to above, and all else has been checked, it is time to check fuel delivery. If even one of your injectors is leaning out, you can have problems. Try something as unscientific as a plug cut, but if possible, get on an exhaust gas sniffer.

If mixture is good, time to check basic engine health w/compression and leakdown checks.