Sounds like it starts idling lean once heated up. My 69 slant ran like that for years. Finally got a good Holley 1920 and it idled unbelievably good. Many bad 1920's out there w/ clogged idle circuits. Not sure why no mechanics could diagnose it, but sounds like the ones you tried are just as worthless. If you haven't yet, change the rubber fuel hoses at the fuel pump and at the gas tank. The old hose gets soft w/ today's ethanol and collapses to starve the engine. Use "fuel injection" rated hose.
Wandering on the highway is most likely incorrect toe-in. That happens as the suspension sags and as the pivot bushings age. To verify, w/ wheel straight, hold a 4 ft straight-edge against each front tire at mid-height and sight at the rear tires. I bet the sight-line will hit inward on the rears, when it should fall ~1" out from the tire. Measure toe-in w/ a tape measure (difference of track width from aft side to fwd side). Should be ~1/8" toe-in. You probably have toe-out, which will definitely cause wander. Adjust the tie rod ends to correct, and equally both sides so the steering wheel stays centered. If smart, you don't even need a shop alignment.
