auto trans shifts too quick into 5th?

Hey all,
While excellerating, when i get around 45 to 50 mph, the rpms drop like it's going into high gear and engine sounds horrible until i get up to 55 or so and the rpm's get back up. It's like the cpu is telling the transmission to switch before it really should and the load is too much for that gear. Does anyone else have this problem. I was thinking it may be back pressure from the exhause making this noise b/c i do need to clean out the cat converter, but not sure. any help is greatly appreciated. it's a 91' b2600i

You have a 5-speed auto trans??!!

It's four speed actually, but I know what you mean when it shifts too soon into OD. My B2600i does the same thing sometimes.....it locks up too quick, if it is a lock-up type of converter....don't know about that.

I don't know what causes it, but I know what your talking about.

So do you just leave the overdrive button off until you get to speed? i'm thinking about doing that. I use to only have it off if i was towing.