3.9 v8 coil connections

My 1995 Discovery 3.9 v8 has had idling problems. On the LT positive side of the coil there are three wires attached. One of these wires leads to a small squat cylinder that is bolted to a plate near the coil and radiator. If I disconnect this wire the engine runs great. Obviously this is not a permenant fix. Can anyone tell me what this component is, and will replacing it be likely to solve my problems?
Cheers

it sounds like it could be a supressor to stop interferance for your stereo and will not do any harm if disconnected although you may here a whistling noise through your radio which changes with engine speed.

After investigation I found this part is a ballast resistor for the coil, which drops the coil voltage from 12v to 9v, except when starting. Disconnecting it let the coil have 12v all the time which it seemed to like, but over a period of time this fries the coil. Next step is to try a new coil and see if this does the trick.