Been working one a 98 chevy PU with a vortec 350. Rebuilt it to a 383. has shorty headers,cold air intake, and a LT1 corvette cam, and roller rockers. Stock ecm. Engine runs with good power but stumbles about 4200 rpm. Sometimes less but never more. Throws the P0339 code(intermittent sig, crank sensor) after about 3 times. Cked all the wiring(tested good) Put it on a scanner and showed normal on all systems except the tach shows over 7000. Dash tach shows mormal but does move around. Have built a new harness from the sensor to the ECM. No effect. Anybody had this problem??? Put this on FSC website but no answers(maybe they don't like new people).