voltage drop stalls suburban

While driving (or parked), my 89 (gmc) Suburban, 3/4 ton, throttle injected 454 will stall. My volt gage will drop, my panel lights come and the radio flickers -- if I'm on the freeway traveling fast, the sub re-starts by itself OR if I'm going slow or parked, I must turn key to re-start. When this happens, I lose everything, including my emergency flashers.
I've cleaned the battery posts, replaced the alternator (and tested it) and checked my main ground. It may go days without doing it and then do it 1/2 dozen times the next day. I recently had my AC upgraded to the newer refrigerant type and my battery has a full charge. Any ideas/suggestions would be greatly appreciated?

I don't know about an 89 but my 86 Suburban has a couple of fusible links down by the starter and I know one of these is the feed for the ignition and the instrument cluster. They are attached to the big +12V wire from the battery. If one of them was intermittent it might cause this problem. They look like a heavy gauge wire (#10 or #12) and come in different colors according to amperage rating, but they are actually a fusible link with a single stranded wire inside. You have to look very closely and bend them and if there is a break you can usually feel it or sometimes you can see a small black spot on the outside of the insulation where it has been hot. These are available at most auto parts stores or in a pinch or they can be soldered back together and heat shrink tubing put over the solder connection.
Good luck...Dave