I have a 1970 351C 4V, dual diaphram distributor. Very few miles on a rebuilt motor. All original equipment except for a new coil, points, distributor cap, rotor, spark plugs and spark plug wires. I also ran a new wire to the + side of the coil. Although the engine idles a little "loppy" it still runs fine at low speed. When I get between 60 and 70 mph is when the engine starts "missing". I've also noticed the tach starts acting strange. I'll run the car up to 70 mph and the tach will be at 3000 rpm then with the engine "missing" the tach will "jump around" from 3000 rpm to 2000 rpm to 2500 rpm all while I'm trying to hold the speed of 70 mph. Can anyone help me correct this problem?