4 spd auto ?

[ggtsvnv]

Hi all and thanks in advance,

I have an 87 shortbed Silverado with the factory 305 and 700R4. The engine was rebuilt less then 4K ago and runs fine. The question I have is my trans is always shifting back into 3rd gear on even slight grades or if I give my truck more gas on the highway it shifts right down to third. It seems like on slight grades or if I give it just a touch more throtle it ought to stay in 4th (od) not shift to 3rd? Is this normal? Sorry but I'm used to driving a truck with a turbo 350 and the cars I have had in the past (V8 Camaros/Firebirds) did'nt seem to shift as much.