1988 4runner has new starter, new battery new battery connectors and leads. Alternator is okay.

This just started recently... when the truck sits for 3 or more hours it won't start, just clicks. Jump start from a charger or another car works instantly. After it has started by jump, I can turn it off and it will start on its own even up to an hour or so later. But after 3 hours it goes back to just clicks.

Took a second battery along in case and tried jumping from additional battery... it won't start. But as soon as I get a jump from another car it starts fine.

Explained the issue to a clerk at Checkers and a customer over heard and said it sounded like a ground fault in the ignition, as the charger and other car used their own independent ground, but the extra battery used the truck ground

I would appreciate any help to get this cleared up, Have tried 2 mechanics so far and they can't find the issue

Thanks

3 possibilities - the battery won't hold a charge, the alternator isn't charging it properly, or something is draining it when it's off.

Just because the battery is new, it doesn't mean it's good. Same with the alternator.

What's the battery voltage when the engine is Off?
On?
On and everything (lights, heater fan, wipers, turn signals, etc) On?