Just bought an '89 IROC with 350, won't run right after it warms up. Starts without touching throttle, idles good @2000 until it starts to warm up (like maybe 3 minutes). Then idles slower and slower until it almost quits (500 rpm) then shoots up to 2500 or so then quits. Starts right up but seems like it's getting too much gas. I can drive it but it's not much fun. Performs better at low rpm's, stalls out if I give it too much throttle. Sensor?