What do you mean shift a little off? I assume you're not bottoming out. It sounds like its drive train related, since it's when you shift funny and driving over a speed bump can cause you to use the clutch differently when easing over the bump.
chevy runs deep, Don't step in chevy!
67 Coupe 289 - TCI C4 w/2k stall; Holley 4150 570cfm; Performer intake; mild cam; Pertronix HEI; 302 heads; Roller Tips; Hedman Tri-Y headers; H-pipe w/Flowmaster 40's; 9 in posi 3.50; 205/50-17F; 225/50-17R.