94 Bravada shifts fine for about the first 30 minutes of driving then it goes downhil

Was driving the other day everything was fine (btw when I bought this thing the original dash cluster was replaced with an analog) after about 30 minutes I noticed a slip in od so I dropped it into 3rd was fine. Third started slipping and I was losing power I could barely do 40 up a hill and when I took off it was like I was taking off in a higher gear no power whatsoever. So I parked it at a convienience store for a half hour got in and it was fine drove it home parked it have been trying to figure out whats wrong with it ever since any idea's?

Was driving the other day everything was fine (btw when I bought this thing the original dash cluster was replaced with an analog) after about 30 minutes I noticed a slip in od so I dropped it into 3rd was fine. Third started slipping and I was losing power I could barely do 40 up a hill and when I took off it was like I was taking off in a higher gear no power whatsoever. So I parked it at a convienience store for a half hour got in and it was fine drove it home parked it have been trying to figure out whats wrong with it ever since any idea's?

You need to get it on a engine /transmission capable scanner and read out any codes and engine and transmission data.
Watch for shift time errows and any sliping.

How is the fluid level and condition and filter?
Any check engine light?
Is engine running good?

Post back any codes transmission or engine.