my truck did exactly that same thing it was the death shake at 45 when the convertor was trying to lock in .mine had a blown plenum .and i eventually after resealing the intake plenum didnt fix the skip i found no compression on 6 and 8 .#6 piston was melted with holes in it and cracked due to excessive detonation probably caused by the blown plenum gasket .wish id done compression test before i bought it .oh well got a new used motor all will be good again.