RE: Diving on downwind?
STLPilot is on the right track. If your Tamecat was flying consistently, and is now lacking in performance,
the only area the problem can be is power. The engine may be running great on the ground, but not unloading in flight.
In level un-accelerated flight: Lift is equal to gravity and trust is equal to drag.
Therfore, the only thing that can cause your problem is a deficiancy in one of the above. You can eliminate gravity and lift.
That only leaves thrust and drag. You can eliminate airframe drag, as nothing has changed there. That only leaves thrust and propeller drag.
The first thing I would try is changing the glow plug and fly it again. (In my helis, when perfomance drops, It is almost always a plug that is worn out.)
The Pico is a pretty hot engine. So, that is the most likely possibility!
Before you go tearing into a perfectly good airframe, Change your glow plug.
If that doesn't do it, change the engine.
Doug