700R4 Slipping in 4th Gear - why?

[mosel70]

Hey everyone,

Got a rebuilt standard 700R4 w/OD from Monster Transmission 2 years ago for my '70 C10 2WD with a '69 396. Open differential. Been a great tranny up to a few months ago. At low speed in 3rd or 4th, occasionally I felt a shudder and lack of acceleration once warm, then it would grab and go. Fluid level always good, shifts fine, everything else good.

But yesterday, about 5 mins into the drive, in 4th about 40 mph, when I tried to accelerate with just normal effort it won't accelerate and the rpm rapidly climbs. Doesn't seem to do that in first 3 gears. I took it straight home. It's still under extended warranty, Monster asked I take it to a local tranny shop here in Las Vegas.

Anyone have an idea what this could be? I really appreciate the help. It's been a great 700R4 til now.