To narrow it down to the igniton comtrol unit, or amplifier.
If you can ride it and get it fail at home, a quick way to see if the unit is failing due to heat problems, when you are at Radio Shack, or other electronics store, see if you can find a spray product that can 'freeze' or cool down the unit, to see if cooling it will solve the problem.
If this doesn't help, try the same thing to the ignition canister, commonly refered to as the 'bean can', located under the front engine cover.
There is a component in there, a Hall Effect sensor, and it can fail the same way due to heat, and age.