could be anything, for starters check all connections for cleanliness and tightness, if the fluctuation seen when running isn't caused by vibration of a poor connection, I'd think it's most likely the regulator, and you might be able to hear that clicking .... and figure out what the meter is actually saying, it's not reading hundreds of volts wink

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics