I just finnished rebuilding the engine in my 83 Thunderbird a few weeks ago. And since then I've been getting vibrations that range in severity with no apparent cause. I get no warning lights but the vibration doesn't feel normal to me at all. I've tried tightening the mounts on the engine and the transmission, messing with the carburetor settings, retorqued the torque converter...I'm running out of possibilities. These are the changes I've made to the stock engine: 1) Mustang cam similar in characteristics to the HO roller cam, 2) E7TE heads from a 1990 Bronco, 3) B&M Holeshot 2400 converter and new stock flexplate, 4) GM style aftermarket HEI distributor running off a relay triggered by the old coil wire from the harness. The vibration is present in and out of gear and it randomly worsens to feel like a heavy thud, then it smooths out again to where it was. I don't feel it at higher rpms though. Anyone have any ideas?