Went for a run today, about 10 minutes after I'm on the highway, my 235 Chevy starts running rough in a strange way. If I give it "the gas" it runs just fine, back off and it stumbles and feels rough. Turned around to come home and after another 10 minutes it was way worse, backfiring out the tail pipes, but still ran well if I kept my foot on the loud pedal. It's been running super sweet for over 12 years!, just got back from a 1200 mile road trip without a hitch and did a few shorter runs in the last couple weeks with no signs of any problems. Now this. 235 with dual Rochesters on Offy intake with water cooling, Fenton headers, Langdon HEI (with fresh cap, rotor and plugs about 2500 miles ago). No smoke of any sort out the exhaust, no oil usage in last 1000 miles, no change in coolant level in last 2500 miles. Pulled the plugs ( they're OK), checked compression (OK), found a small vacuum leak (fixed), pulled the air cleaners off the carbs, found a lot of liquid gasoline in the rear airfilter, tightened up everything on the carbs, manifolds, etc. nothing in particular was loose. Idles "okay" but I can tell there's a miss in there somewhere cause she's shaking a bit and a bit of a pop out the exhaust. Any clues? Sending it to my mechanic on Monday.