rough idle on 86 22r

my 86 pick up is doing some very strange things since i took it on a 400 mile trip. it started stumbling on the freeway and wouldn't accelerate. then after pulling over a couple times to check it out, it ran great, but the idle is really rough. it melted the electrode on the #4 plug. the O2 sensor, wires, cap & rotor, and plugs have been replaced, the valves have been adjusted as well, all to no avail. it's a 22R carbureted, a fresh rebuild with 25K miles on it. even the guy who built it can't figure out what's wrong.vacuum leak? bad valve? i noticed it makes a slight whistling sound when driving. i was also driving in the rain when this started happening(this may have something to do with it?). i am fairly handy with a wrench, i just can't diagnose this problem, any advice would be greatly appreciated.