904 Problem

There should not be any "flare up" between gears, which is what it sounds like you are describing. My first suspicion is the forward clutch, which is engaged for all forward gears, but yo don't mention any problems in starting out from a dead stop. Does reverse give you any problems? If not, the direct clutch is likely ok.

How much more power are you making? A 2800 stall with one engine will act like a higher stall with a more powerful engine.