Mooney Low Voltage

Can anybody help me diagnose this problem? Just bought my new (to me)
1978 Mooney 201, flew it back from Kansas City to Atlanta, no problem.
Flew to Richmond, VA - again no problem. Leaving RIC for Atlanta, the
low voltage light started flashing. Didn't want to carry on IFR so
stopped into Danville. Left the plane there, and the mechanic removed
the altinator and tested it (all ok) and replaced the voltage regulator
with a new one. All ok (charging 13.9 volts). Started the plane, the
altinator gauge was showing a charge, and took off. About 30 mins into
the flight again the low voltage started flashing. By the time I had
checked all the CB's it stopped. Then about an hour later it started
again. I tried pulling the Alt Field CB and resetting, no joy. It
continued to flash untill I was in the pattern at LZU.
So, now - what next? Bad Diodes on the Altinator breaking down at
altitude?(I was at 8000'), loose altinator belt? (unlikely - it had
just been reinstalled). Over voltage on the Altinator causing the
Voltage reg to drop off line, and I didn't wait long enough when I
reset the Alt-Field circuit breaker? Bad ground straps??
Any and all thoughts welcomed at this point - this is the worst kind of
problem to hunt down.
Thanks
Ian