Intermittent electrical problem
 

For the past 2 weeks I've been experiencing a problem that's been difficult to diagnose. While driving I'll get these 3 dash indicators on; Brake, Charge & AT Oil Temp. A few seconds later they go off. Sometimes they stay on for up to 2 minutes, but do eventually go off again. Of course it never happens while parked. It appears from the schematics these 3 lights are common to each other and to the 'L' terminal on the alternator. Static battery/alternator testing doesn't show any problems but I'm wondering, can an alternator work/not work like this? Is the alternator the place to start looking for this problem? They're not cheap and don't want to replace it unless necessary. Truck starts and runs just fine otherwise.
'86 2.4 EFI auto.