Took me 6 months to find a tiny vacuum leak on my 1966 Mustang, and it was a $3 hose
I chased a rough idle on my 289 for half a year, replacing the carb, plugs, and even the fuel pump. Turned out the rubber line from the intake to the brake booster had a pinhole leak that only showed up when the engine was warm. Anyone else ever get beat by a cheap part after throwing money at the wrong stuff?
A $3 hose after six months of throwing parts at it? That's the classic old car tax. Half the time the "big" fix is some crusty piece of rubber you could've swapped in ten minutes, but you never find it until you've already blown a paycheck.