Sluggish 904 when cold or converter?

Hey guys,
I've owned this car for two years and when I fire up my 75 Scamp (225/904) and I shift into reverse to back out of my garage it harldy moves as if it has a high stall converter. It's not slipping in and out of gear, it's just real sluggish. The Trans NEVER slips, the fluid level is good, no leaks, and it only does this after sitting for a while and cool. What do you guys think would cause this? Thanks!