Th350 slips at full stop.

[77chebby]

my th350 in the 79 silverado slips at full stop sometimes. never slips changing gears and rides great in 2nd or 3 rd but when i come to a complete stop it revs a little before clunking or chattering a bit(under hard acceleration) and moves, then its fine. only does this after its warmed up. i changed the filter and fluid and gasket (it was leaking enough that it was hard to tell if i was keeping the right amount of fluid in it.) . the pan still leaks pretty good, could be warped or something. anyways any ideas on the slipping ? only does it at full stop after its warm. thanks guys , mark