'89 B350 transmission problem

[DodgeVan89]

hi,
i have a '89 dodge ram 350 cargo van with a 5.2 L V8. I'm not sure what transmission is in there but it does have 3 gears. it has a throttle body carb.
The problem is:
the first time accelerating past 35 after starting the engine. when the transmission tries to go in to third gear. it goes in and out of gear. I can get it to stay in 3 gear by holding the gas peddle at or near full throttle (i know its in third gear because i can shift down to second). when it gets up to about 50-55 mph. I let off the gas and feather the gas peddle back down to maintain speed. It continues to work fine until it sits for more than about 4-5 hours then the problem starts again. It only happens the first time accelerating when transmission is cold. Any help would be great. Thanks

[alloro]

What was the last time it had a fluid/filter change and the bands adjusted?

[DodgeVan89]

I read the manual I ll give that a try...