Roland...don't forget...don't rule out that it could be a vibration issue,Bud.I had that going on for quite a while...popping bulbs.I eventually narrowed it down,and after making more grommets for the headlight bucket,where it attaches to the ears,and I even applied some silicone to where the H-4 bulb gets inserted into the headlight...since I did all that,I haven't popped a bulb.
At first,I thought it was electrical,but even after I upgraded the reg/rec to the solid state one,it was still happening.It was the following season that I went for vibration.I'm pretty sure that I have it,but Who knows for sure...it's one of those things,I suppose.Everything else was tested,between Myself and Others,as far as "Spiking" went,and also with a new battery,I hooked up a multi-meter to the battery,and ran the engine at different rpm's,and watched for any serious changes in the voltage output(I used an analogue tester...easier to see sudden changes).
If You've tested all the electrics...take this into concideration,Bud...good luck.