'99 v70xc, intermittent cruise control

the cruise control on my '99 v70xc (128k miles) has recently started
to behave oddly.

sometimes when i try to enable cruise control, nothing happens.

sometimes when i try to enable cruise control, cruise control kicks in
(and the cruise indicator is on for a handful of seconds), but then it
drops out (and the cruise indicator turns off).

other times, cruise will have been happily engaged for at least a
short time, but then it will suddenly drop out.

once cruise control does fail, it usually won't go back on again right
away, but frequently will engage again after another 10 or 15 minutes
of driving (without stopping and restarting the engine).

i've already had the ETM replaced (twice, first out of my own pocket,
which volvo ultimately reimbursed, and then again under warranty less
than a year because the first replacement started failing again), so
i'm hoping that's not the problem yet again.

the check engine light is not on, and i have not (yet) asked my local
mechanic to check for any codes that may indicate the problem.

any ideas about the root cause and how to fix it? (e-mail replies are
best; if there is sufficient interest, i will happily summarize back
to the group.)

thanks,

kirk
(note trivial anti-spam hack)