Another opinion from one that has used the ferrite rings is not to bother. If you really want to try them pay the postage and I'll send you some rather useless extra weight.

Moving on, it's possible that the ground location specified is not functioning properly. Try moving the ground wire to another convienient location of the engine. Check to see that the wire is secure at the ignition end. Check that the spark plug cap is as far down on the plug as it can go. More important, call Tower in the morning to get their take on the situation since they are the distributor. Try lightly tapping on the rx with the engine off to see if you get any glitching, indicating a crystal, antenna, filter, or other rx issue. Do the same with the tx though I don't think you will come across anything there. Change out the switch and the battery, one item at a time, to see if the problem is solved.

Most important, though time consuming, is to only try one item at a time and then run the engine for a very brief period.

The most common causes have been pretty well covered in all the posts so far. Michael Glavin's suggestion of posting several clear and detailed pictures of your installation is a very good one. Trust in his judgement of what he sees. He's very good at what he does.

Good luck,
Pat