I would check the tape first, too. It's difficult to believe the impact
a small piece of loose tape can have...or the difficulty of finding it.
Many years ago at the U.S. 15M Nationals (why do these things always
happen at contests?), my LS-3 began to pitch down abruptly when
thermaling. Just enough to get my attention. It didn't appear to stall,
but the nose dropped enough to make me wonder.

After a while, I noticed this only happened when I was carrying water
ballast. Day after day, as I talked to experts, I'd take off with
water, find myself feeling uncomfortable in gaggles, and finally dump
ballast to fly the task. The problem didn't always show up
immediately--sometimes it was 15 to 30 minutes before this started--but
it was always there eventually.

Finally I discovered that a 4" wide section of cloth tape on the top
side of the elevator hinge joint was not stuck down firmly. Apparently,
at flying speeds the front 1/4" of the tape would eventually work loose
and flip up intermittantly. On the ground it looked normal (and the
vast majority of it was). The only reason I found it was running my
fingernail along the front edge of the tape. The sticky part had dust
on it, indicating it had been in the airflow for a while.

The fix was easy: I simply applied the usual white plastic wing gap
tape to the front edge of the cloth tape to hold it down until I could
replace it, with immediate results.

Chip Bearden
ASW 24 "JB"