I really wish i could give you s straight answer...but..........

I had the exact same problem as you........

And spent ages trying to diagnose the problem, one bit at a time.
Was getting nowhere in the end, and was getting fed up so ended up changing loads of bits at once...........and cured it. So i'll never know what fixed it.

But here's what i changed.
New CAS,
New Ign Amp (i thought this was the suspect)
New Phase Sensor

Probably of little use but hope it helps in some way.......