RPM ON ENGINE

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I HAVE A 94 BUICK CENTURY 41,400 MILES. HERE RECENTLY. I CAN REV UP THE ENGINE AT HIGH IDLE, MAINTAIN THAT IDLE. AND THE ENGINE RPM WILL GO UP AND DOWN, UP AND DOWN. ENGINE LIGHT, IS NOT ON. I'VE USED MY OBD AND SHOWS NO CODES. COULD THERE BE A VACUME LEAK SOME WHERES. PLUGS ARE NEW. DRIVING DOWN THE HIGHWAY AND STEP ON THE GAS, IT SPITS AND SPUTTERS. NO BACKFIRE. I'VE HAD THE INJECTORS FLUSHED OR CLEANED. NEW GAS FILTER. NOW WHAT? TIMING?