Out of the blue my 1986 245 (333340 miles) has started stuttering randomly. While driving at cruising speed the tachometer will jump up to like 2K and drops to 500 or even 0. It feels like the motor stops briefly for a split second. Sometimes this occurs once, other times it will occur three or more times in a row. I noticed too that the temp gauge is zeroed out too. I am not sure if I am having electrical problems, fuel problems or something else. I have recently replaced the plugs, rotor,cap, plug/coil wires, fuel pumps, and timing belt just as routine maintenance. Any ideas?

Thanks

 

The tach is the clue that its ignition/elec , not fuel delivery.
The tach indicates ignition, if it drops to zero whilst the engine is still rotating the ignition is dead, if the tach dropped slowly with engine rpm it would be fuel.
Coil ground can be a problem.

 

Thanks! Which terminal is used for ground?