Intermittent electrical charging problem

I've searched and haven't found anything on this problem. I have an 89 XJ automatic with the 4.0 in it. Here's the problem about 3 months ago my g/f was driving my jeep and when she got home said there was a problem with the radio so when I looked at it the battery was almost dead so I charged it with a battery chager check all the connection + battery, ground etc.. all good so I took it to get the alt & batt check, all good. No more problems until yesterday I was driving and the radio started going off and on looked at the battery gauge and it was slowly draining stopped at a store and had to get jumped to start it so I headed home and all the sudden the battery gauge jumped to 14v where it usally is then when I was almost home same thing the gauge dropped to about 9v and my jeep started cutting out just made it home and the batt was dead. Charged it over night and today it's back to 14v and seems fine. Any ideas??

First check for corroded connections, frayed or shorted wires, etc. in the harness to the alternator. If that isn't it, a likely suspect is worn brushes and/or dirty sliprings (which often go together, as worn brushes lose tension and allow the sliprings to scorch or corrode). This will usually cause intermittent loss of charging, which will get worse and more frequent until it stops altogether.

If you're there when it happens, try taking a stick or other object (careful of moving parts, belts etc.) and hit the alternator while it's running. If it momentarily recovers, I'd suspect bad brushes.

Had similar problem with my 95. I'd notice drastic voltage drop in gauge, then would rev it up, and it would hop back to 14v. Changed out the Alty. and runs great now.

I followed Matthew's advice and when it happened again I beat on it and sure nuff it started working again. I replaced it and haven't had any problems since.