2004 merc 50 4 str efi low power/rough idle

[lunker1000]

My engine has a rough idle-bounces from approx. 800-820rpm. I finally got her out a couple of weekends ago first time all season, put in fresh gas,can of Seafoam, new fuel filter and plugs. I could not get the motor past 3200rpm at wot-13mph.
Last year she ran beautifully 5500-5800 rpm at wot 33-35mph. I winterized it-changed oil/filter, gear lube, put Stab-il in gas-ran it through and removed spark plugs and injected some oil into cylinders-just like manual says.
I can't figure this out- Fuel sight bowl seems to be getting gas, starts no problem.
Any advice/help appreciated.
Thanks

[JUST-IN-TIME]

plugs could be fouled up

[lunker1000]

Thanks for your response. Any help is appreciated.
I took the plugs out seems to be burning fine-changed them anyway, they've never been changed. Started it up in the driveway still has a rough/bouncy idle, haven't brought it to the lake to run it but I would guess that it hasn't changed anything.
Thanks again.

[JUST-IN-TIME]

might need to plug it into a ddt
could be a bad fuel injector