Charging problems

Hey All,
I bought a 1994 Mustang GT 5.0L 5spd. 3 months back.
The old owner said he just replaced the alt. and batt.
3-4 weeks ago this problem started, where it would stop charging while I was driving. After a little driving it would kick back on. Its been doing that now for about two weeks off and on really bad. It happen in the begining and when I revved it up like 4k-5k rpm it would kick on. Now when the batt light comes on and the volt gauge drops no matter what I do it stays on and doesnt charge. I have put a multimeter on it to make sure it wasnt charging. When the light comes on it will show 10-12 volts. when its working fine it shows a little over 14.
When the light comes on and it stops charging it will usually come back on after 1/2 hr of driving, or I have to park it let it cool completely down and it will be fine. I have checked the engine ground from engine to batt, batt is good. Any help would be great. Car has 108k miles. All stock.
I will check the wire going from the starter solenoid tonight, someone said that may be burned.

Thanks all

Robert Johnson

charging problem

When it stops charging it does it for long periods of time, even at idle.
And I checked the belt its not slipping.

Johnson