1st Gear Slip

[COULDA HADA V8]

I had the tranny rebuilt in my '87 GN about 5 years ago. I've only putr about 3,000 miles on the car since. It was done at a reputable shop with all Art Carr internals. The tranny shifted a little hard for my taste but was acceptable. I just took the car out for the first time this year and it has developed a low gear slip under full throttle acceleration. The fluid level is fine and the color/odor is good. Any ideas where to start looking???
Thanks!