89 octane question (Help Zooker please)

[skeeterman]

I have a yamaha 1989 50hp motor.I have been burning 87 since i bought it a couple months ago.But yesterday i needed fuel while i was on the lake so i pulled up to the pump at a store on the lake.All they had was 89 and 92 octane so i used 89.Well at the end of my fishing yesterday.My boat stated cutting back alittle at full throttle.So today when i went fishing i fueled up with 87 octane.But there was still 89 octane in the tank around half a tank.Today i could really notice the cutting back and idling like it wants to cut off a few times it did.Loading the boat on the trailer it seemed like it didn't have the power to go on there.My question it do you think i may have a fouled plug since i changed the octane.If not can anyone give me there opinion on my problem thanks guys.