Help-amp voltage problem

Ok I have a 88 5.0 5speed car, here is the problem, The volt gauge (stock and autometer) usually reads 13.5- 14 volts thats at idle (around 1grand) and driving. But after driving for awhile the car when just idling seems to idle down to 700 rpms and volt gauge drops to 12 volts. Now it doesn't always do this. When it does drop the car sounds terrible. It just seems like somthing puts a load on the car and it drops. It has a new battery and alt, actually it has been doing this fora couple years not so its had a few batterys and alts. The battery is still up front and both terminals are new. It doesn;'t idle surge it just drops a little and sonds bad. Any help is appreciated. thanks

Cold idle is good and loaded is usually good also, this just happens every now and then. I did change the IAC and I have cleaned the mass air meter. It is a 88 car but I cahnged the engine harnes to a 88 Cali cars for the mass air swap. Thats been a couple years ago, It has no pollution or AC stuff on it and stil runs strong just that idleing down and sounding bad is driving me crazy.

Set timing today, it was at 10deg and I bumped it to 14 seems to run stronger and wasn't idling down but we will see.