1990 Ford Mustang 5.0 GT headlight issue?

So i know foxbody's are notorious for their foglight/headlight issue where they would flicker on and off like a police car when running both the foglights and the headlights at the same time. But this is not my case at least i believe so.
My issue that that my headlights will work but i have to find a "sweet spot on the switch", other wise either just my headlights will be on and that's it, my tail light will be off as well as my parking lights. It is hard to keep the switch in the position that all my lights will be on and function, ill usually hit a bump or something in the road and they will all turn off.
The parts that i have replaced consist of the headlight housings, all bulbs, multination switch, headlight/foglight switch, the foglight and headlight connectors.
This car has been sitting for a long time before i got it. So many parts have been replaced, but i've had this issue since i first got headlights on that car. The previous owner did not have this issues, and no hes not lying about it, hes a family member and gave it to me.
I hope i was specific enough for a solid answer, i want to thank you in advance and any help will be greatly appreciated.

You need to check the headlight and fog-light GROUND wires. Consumers Reports must have been right. They said Fords have terrible electrical systems.