Sluggish Acceleration

Hey guys, looking for some ideas. I just had my transmission rebuilt and radiator replaced on my '83 Celica GT. It drives perfect to start, but after about 1.5 hours of straight driving today, it got sluggish to accelerate in the 2-2500 rpm range. But drove perfectly again after I let it sit for 30 minutes and drove again.

Any ideas? I'll look into my filters, but not sure what would cause it to be time sensitive sluggish.