If it still has the cat on it I'd be for cutting it off and taking it for a short ride around the block.. It sure sounds like a stopped up exhaust from here.
Loosing rpm is hard to do suddenly except for exhaust or fuel. Reving to 4500 at stand still means the engine is prolly good.. so that leaves not enough fuel or NO WAY for the exhaust to exit.
Remember it takes more fuel and makes more exhaust when moving.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist