Agree with steve for the checks
I'd have a look at all the plugs on the reg/rec, there are 2 which on mine could come apart quite easily and any connector associated with the yelow wires, they carry a lot of current and the connector can fry as I remember
It's also an idea to add an extra grounding strap from the reg/rec to a good solid ground.
Otherwise I'd start as steve said with the AC output to Reg/Rec from the Alternator then check the DC output from the Reg/Rec. Remember to check the DC with a fully charged battery as a discharged battery can pull the voltage down