check the alternator output, should be ~ 14.5V with the engine running, on mine ( where teh alternator had recently gone) when we disconnected the battery it dropped to 8-12 V)
There is also a connector on one of the chassis legs that I believe gets all corroded up. Check its all clean and good.

Are you SURE teh battery is good, I had similar when my battery was dying, clock resetting, sometimes starting fine other times wouldn;t even turn over and irrespective of how long it had been running or even how long since I'd turned it off.