'86 Nissan Hardbody Pickup Warning Lights Malfunction

After starting my truck the dash warning lights (oil, battery, brake, low fuel) stay lit and flicker for 20-30 seconds then go out with the exception of the battery light. It remains dimly lit for 2-3 minutes before finally going out. The battery and alternator have tested OK. I cleaned the battery ground connections and
replaced the check light relay. Still the same situation. Any suggestions? Paul Halesworth

If there is a external voltage regulator then that could be causing the issue otherwise, the alternator is most likely causing the trouble, even though it checks good on the bench.