>What I find interesting is that with a short delay or none, I sometimes get several dropouts in a row. So my guess is that there's some windows service that occasionally blocks the access to the clipboard for a very short period.

Could be an external block. I saw the same pattern (often 1 or none, then all working) but visualized
all working to 4 x 400m sprint passing staff
none working football player carrying the ball is clipped to th ground, everyone jumps on
(the cleanup of many functions will block)

regards
thomas