On my '87 auto vert, I usually have a high idle in park (and drive & reverse). It runs at or slightly above 1500 (occasionally up to 2000) in Park usually. Sometimes, though rarely, it settles at 800. If I rev it, it settles back to where i was almost all the time. How can I fix this high idle? It's burning gas that I can't afford, I'm sure. Thanks...