Not sure exactly what you have a s far as your charging set up, but if it is relatively stock it will not charge well at idle. Converting to an internally regulated 3g alt can help improve this and reduce your failure rate.
On my '88 my voltage will vary at idle from the 11.8v range to 13.5v, depending on what load I have on the system. Mine is a van with front and rear AC. If both blowers are going it really pulls the voltage down. As soon as I take off the voltage jumps to the 13.8-14.8v range. If I have been driving long enough to charge the batteries well it will sit over 13v at idle with no load.
The stock charging systems on these vehicles are not that great. That is why people tend to upgrade to a 3g set-up if they are going to tax it at all with added components.