You didn't mention ... does the problem sometimes/eventually clear itself?
Last season I had a similar (intermittent) experience caused by a sticking
carburator float. When the float stuck, excess fuel would overflow into the
engine causing rough running and stalling when put into gear. I would have
to increase the idle setting by adjusting the idle stop screw just to get
underway. And for whatever reason, the problem never occurred when I first
started out for the day ... only after running for as you say a 1/2 hour or
so. If when you stop, and you're carburated, pop the hood at that first stop
.... if there's a strong fuel smell (in my case fuel could be seen leaking
out of the carb onto the manifold due to remaining fuel pressure and stuck
float) a stuck float, or dirt/worn needle/seat is likely the problem ... my
2 cents.

"sudsy" wrote in message
om...
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.