The radio has a issue..it should work correctly above 13.6v. The rotax generator is horrible for spikes and the rotax regulator is nothing spectacular..they get really hot internally and get bad joints on the bridge they use inside this brings all sorts of issues like noise and spikes as well. The best way to see if its your radio is pull it out and set it up on a powersupply on the workbench with a clean DC source then raise the volts and see. This will tell you a lot then. Most regs are putting out 14.2 or so volts when the engine is running . The other thing to check too is the 22uF cap that should be on the output of your regulator. if its old it could be snookered too and that helps stop spikes and even though Rotax say that is a option it really needs to be there all the time but like all caps especially ones that are getting hammered wont last too long.