93 voyager starting/electrical problems

Started as an intermittent problem that got worse & worse until no starting....just a click. Replaced Starter, and started immediately...thought we were good to go. Ran for two days, now the intermittent starting problem is back again. Battery & Alternator tested good, but we see and hear weird electrical oddities when it doesn't start, like...radio display comes on but no sound. Turn head lights on and it makes a whiny, buzzy sound. Step on brake pedal & the radio sound comes on! Sounds/looks like voltage/current issues. Battery measures 12.5V....specific gravity check is good in every cell? Appreciate any suggestions?? Actually just trying to fix so we can sell. Van gave us 147K very reliable miles until now...