My 1993 Aero 5spd w/ TCS 67k has had occasional rough idle for a very long time. It only happens when the engine is hot and never when the A/C compressor is running. The idle speed is correct, it just runs rough. The DI cassette is good, head gasket replaced, fuel and air filters, new plugs w/ various gaps, all new silicon vacum lines except the large brake booster, plenty of injector cleaner run through it, throttle body clean. The car sits for long periods and is mainly driven on long trips at high speeds. Any ideas as to the cause?