Buick 4T60E trans lock-up problems

I have a '96 Buick Regal 3.8 with 180k miles. I've changed the tranny
fluid/filter several times in its life but since this last change I
seem to have a problem. The trans will shift normally during
acceleration hot or cold, light or heavy pedal but once the lock-up is
engaged it will unlock and relock with a slight push on the pedal.
When I say slight, I mean, for instance, the slight fluctuations of my
foot when hitting a moderate bump in the road will cause it to
unlock/relock. Other nuances: a very slow and steady pushing on the
gas will not cause the problem.....a decent push on the gas at the
base of a hill will cause it to unlock but it will stay unlocked
appropriately for the load going up the hill....it will do the same
thing in overdrive or '3'. I have a tach so I can see exactly when
the unlock/relock occurs. Sometimes I get a shudder when the trans
seems to be unsure whether to relock or not.

When I was changing the fluid I wiped off the gasket surface and the
rag caught on a piece of metal about 1/4" wide 3" long with a 'V' bend
in the middle and knocked it off. This was up next to the filter. I
put it back on the way it made sense i.e. with the 'v' pointing up
holding another piece of metal in place against the valve body (maybe,
I can't picture it in my head now).

Any suggestions as to what might be causing this odd behavior? And
did I put that thing back on right?

Ron