Regulator gone bad?

Full fielded the alternator and verified that it's charging at the battery. Swapped in a spare regulator and now the voltage comes up on the volt gauge in the dash with an increase in RPM's. Cleaned up my ground points on the suspect regulator and it works okay too. I've got between 12 and 13 volts at the battery at idle.

Also took the battery to a battery shop and had them load test it. Passed just fine.

I could get my electronic speedo to "turn on" and work by bringing up the RPM's from idle. As soon as the volt gauge hits 12v or more the speedo turns on and works. If I let the car idle it drops to 10v and the speedo shuts off.

I measure 12+v at the battery feed on the fuse box with the car off. As soon as I start it, it drops below that.

I recall seeing voltages below 12v at idle for the past few years but the speedo continued to work fine. I've got to figure out why and where the drop is occuring. Have already looked at the bulkhead connector.