Troubleshooting: Engine Loses Power Under Acceleration

My 1996 Volvo Penta 5.8 starts great every time. HOwever, once on plane and running a minute or two at cruising speed then starts to chug, badly also leaving a rich fuel smell of exhaust like snowmobile exhaust, once you cuy the throttle it will wun fine, then if you open the throttle back up it will eventually chug again. So far, it has not stalled, however it definately sounds like it is going to.