Mercruiser starting/running problems

[RoisandPhil]

I have a 220 hp 1999 Mercruiser 5.0 EFI. I took it out for the first time this season last week and it ran well for about twenty minutes until I backed off the throttle to about half way at which point the boat stalled. It took about ten minutes to get the boat started again and I managed to labor into shore. The boat engine was running very rough with a bit of vibration and once or twice sounded like it may have misfired. It felt like the boat wasn’t getting any fuel. Anytime I would touch the throttle either up or down the engine would stall. I was lucky to get back to shore at a constant throttle setting. I changed the water separator (no sign of water) and cleaned the flame arrestor. Back on land connected to a hose I managed to get the boat started again and it ran fine while adjusting the throttle up or down for a while. Then the problem started again and immediately after starting I would have to give it considerable throttle to keep it running. Again after running, if I dropped the throttle the engine would just die. I am pretty sure it’s not a contaminated fuel problem as I dumped the contents of the separator into a glass jar to look for separation. Any help would be appreciated.