97 Tbird 3.8 Intermitent loss of power in 1st

I have a problem accelerating from a stop after the vehicle has reached normal operating temp. There is a severe loss of power in 1st gear. After the trans shifts into 2nd the vehicle runs better but still lacks the power it should have. It can run great for days and then all if a sudden the power is gone. Let the vehicle sit for a while and it will run fine again for a few days. I have noticed that it seems to run warmer after the power loss. I have replaced the O2 sensors, MAF sensor, New Plugs, Fuel Filter, Air Filter and Injectors checked and cleaned. Vac hoses have been checked and are fine. The Tbird has 82,000 mi.V6 3.8. Does anyone have any Ideas??? Tom