Is this when it's cold outside? All cars have an idle system that adjusts the RPM to warm up the engine. The early systems in the 80s/early 90s are pretty archaic, so that could be it. My '86 Integra would rev it's engine for about 5 minutes on a cold day to heat up and if I put it in gear, it would actually coast like an auto even though it's a stick. Either that or a broken thermostat. Either way, fixing it isn't going to be fun, so I'd say take it to a shop, they can figure it out.
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