Try pulling the cluster and taking a small Phillips head screw driver and tighten up the screws on the back of the cluster that hold the plastic grid. Don't crank them down just snug them up. The alternator needs a signal from the cluster to get it started charging. My 87 745T did what your describing in your post. After reading the FAQ on 700's I as stated above and the problem went away. What is happening is your running on the battery alone. In my instance it happened over the winter and as soon as the interior warmed up the problem went away. So far this winter I haven't had a problem. If you think it may be the alternator take it to a local Generator/Alternator re-builder and have it gone over. It should cost less than $100. The reason for the local re-builder is the cartridge with a Voltage regulator and new brushes is around $70 at the dealer. For $30 more you get the whole unit rebuilt including new bearings with a warranty. I suspect with the mileage you have that the brushes are worn to the point of needing replaced.