700r Tranny issue

[Nukes16]

Hey guys, I'm hoping one of you can help me out. I finally got my new winter baby (86 short-fleet) on the road but it is struggling to stay in 4th gear. When I'm cruising down the highway at about 60 and I've barely got any pressure on the pedal it keeps shifting down to 3rd instead of staying in 4th. It seems to happen on straight, level sections of the road.... when it should be just lugging along. It will go back up into 4th if I accelerate a bit first or chop the throttle completely. I've checked the tranny fluid and it is up to par. Any suggestions would be great!
Cheers