This one totally has me stumped. It's my 68 bird, 400, 4-speed. I purchased it this summer & finally got to start driving it after doing the interior work & some engine compartment stuff. It starts fine, but when your driving at cruising speed the car hesitates every now & then, not where the car shakes & buckles, but just enough that you can "feel" it. Here's the set up. cam: unknown, the guy I purchased it off of had no clue what the cam is but it does have 4.11 gears at the rear. I replaced the oil pump, water pump, new MSD 6AL box, new MSD distributor, new Holley fuel pump, new Holley 600 cfm double pumper, new fuel filter, new spark plugs, new MSD coil, & new spark plug wires. It has an Edelbrock Torker intake & headers. It did have a 670 Holley Street Avenger carb on it & it did the same thing. I thought it may be the carb & got the new 600 & it didn't cure it. I've even tried another 6AL box & that didn't work. In order to make the engine start/idle/run I have to have it about 18 degree's advanced, so it must be a serious street/strip cam. Starts up fine, idle's perfectly fine (around 800 rpm), & if you punch it there's no miss at all. The miss is only present at cruising speed, which is around 3000 rpm for 55-60 mph with the 4.11 gears. So...any idea's on this one. I'm completely out of ideas. I've talked to the MSD & Holley tech guys & they all said "something's not right"....no duh. I've heard everything from fuel to rich, fuel to lean, the cam jumped a tooth, a cam lobe is worn down, & a few others. I may over the winter just pull the 400 & tear it down & rebuild it but before I do that I figured I'd run it past you guys while theres still a couple weeks left before I put her in storage. One other thing, it runs hot in stop & go traffic....it'll soar up to 215 degrees. I do have a 160 thermostat in it with an aluminum radiator.