My ride had the same thing going on. i have a 91 accord with a f20a engine. same thing, basically... anyways, it would rev high and low in park constantly until i drove off. the honda folks here checked it out and said that it was a vacuum hose that had collapsed. i'm not sure where it was, but if i remember right, it was in the back of the block. now my ride is working again... might wanna check it out. hope it helps... good luck.