The special triangle shaped connector that hooks to the VR is a common point of failure. The connectors get dirty and hot then open up. Changing the regulator may have temporarily gotten a connection.

If the connector looks bad, the help section at the AP store usually has replacement with pig tail that can be spliced into the harness.

PS: Measuring voltage at idle is not a reliable check of the system. Alternator output is low so depending on lots of factors (loads, pulley size, idle speed, battery ect.) voltage may be quite low. Best to measure at around 2000 RPM with a charged battery so you are really seeing performance of the charging system.