I started out with a Bell wireless speedo (walmart).It punked out after about 150mi.Upgraded to a Cateye with a wire which worked fine for about 300mi.than got a mind of its own.I'm out $50.00 total and pissed.Not quite broke yet I bought some ferrite filters off Ebay.Turns out The Cateye snaps into a base unit and has 2 small contact points.I polished these points applied a little vaseline and that fixed it.Got 800mi.on it.
To get to my point,I bought a 5 pack of these filters.I don't know if there's anything to this interference theory,but if your convinced you need one send me a PM.Free to first 3 responders.Ron