Alternator Light erratic behavior

[EsGeht]

It's a Marchal in a 72, and had a bad diode repaired about 10years/40,000 miles ago. Since then it's been perfect. No recent electrical work has been done, and batteries are fairly new and kept on a trickle charge. Fan belt is properly adjusted. During a 40 minute freeway drive this Sunday, at steady rpm, the alt light began to flicker faintly, come on full red, go out, come back on, go out, come back on, etc. Revving the motor would sometimes cause the light to go out, and sometimes it wouldn't. Back home the next day the light continues to be goofy. I did manage to get readings, but as I watched the multi-meter I could tell whether the light was on or not, i.e., readings were also erratic.
Battery: 12.78
At idle: 12. 71 (faint glow) but then the light would come back on and reading would drop
3,000 rpm: 13.97 when the light was off, but it fell into the 12s when light on
I get it if the light stays on, but I don't get this on/off behavior. I'm ready to pull the alternator and have it repaired if it is truly bad, but before I do, has anyone seen this erratic alternator light behavior? Ideas?

[304065]

Yes, this is common behavior when you have a bad voltage regulator. Replace yours with a solid-state beru and save the original for show.

[EsGeht]

Thank you! The original is long gone, in it now is a Huco, and replacements are available and not expensive. I'll probably go with another Huco as I cannot find a solid-state Beru. At least I'm pointed in the right direction, i.e. the electrical panel and not the alternator. Much easier to replace!

[EsGeht]

I found Beru VRs for Bosch, I need a vr for Marchal, and Huco seems to be the correct replacement as per PPs catalog.

[EsGeht]

Well.....I replaced the VR with a Huco (same as was in it) and erratic behavior continued. Then took it for a ride, and after more on/off/flicker/glow etc. it came on full red and stayed on. Looks like the alternator is coming out for a rebuild.