I've got an off idle stumble I'm trying to fix on my Carter AVS 383 auto & I put a fuel pressure gauge on it to see what it's getting. It shows about 4.5 psi on cold start, but then slowly drops down to about 2 psi by the time the engine is warm. Revving the engine makes no difference. It's one of those Jegs brand gauges & I've never used one before. Any ideas? Can a mechanical fuel pump actually lose pressure as the engine warms?