TH400 problem. please help

[Cody_454]

i have an 89 gmc one ton 4x4 with a th400. and when the truck is cold it shifts just like normal. but when it gets up to about running temp it will not stay in 3rd gear unless i shift it manually at about 50 mph and then keep it above 60.. i changed the vaccum modulator and still no luck. any insight is helpful. thanks