Could be a worn O-ring near the handle-end of the dipstick... this was the
case on my '82.   Very little blow-by would pop it out. Your model may be
different, but it's worth checking.

>I noticed my 88 weekender dipstick starts popping out of its tube
>everytime I hit the freeway... I remember there was a link about the oil
>dipstick being popping out of its tube... I forgot what caused it???

ChrisR