Auto transm.? (NAC)

[Tihol Tiholov]

Sorry for NAC.
Patient is 1990 Ford, auto overdrive transmission.
Changed the transm. fluid 2 days ago.  New fluid is synth, from wal-mart.
Had to re-use the filter, none in stock at FLAPS.  Missed to re-install a
small square gasket, under 2 sq. in., sitting on top of filter (found it in
the old fluid when discarding).  Old fluid looked good, smelled good, tasted
... well, didn't go that far, no visible metal particles found.
The fluid level was good on cold transm., when checked at idle as
prescribed.  Haven't check it with a warm transm. yet., too little driving.

Complaint: Happened twice in 2 days' total driving of about 30-40 km - Under
acceleration from crawling pace, the transm. seems to slip.  Engine RPM
climb, very little forward motion, then BANG!, the transm. abruptly shifts
into gear and off we go.

All advice appreciated,
TIA,
Tihol