Could be the coil and/or plug wires breaking down once it gets hot.
Could also be the carb needs a rebuild.

--
Tony
my boats at http://t.thomas.home.mchsi.com

"Matt Lang" wrote in message
om...
(sudsy) wrote in message
. com...
need help. my 1990 mercruiser 3.0L runs fine for the first 1/2 hour
or so. then after it sits while we swim or change skiers it starts
and revs fine, but runs very rough and even stalls when put into gear
and given gas. Not a fuel or fuel system problem since it always runs
good for the 1/2 hour or so. this problem is driving me crazy. any
suggestions welcome.

tank vent clogged?