2002 Mercury 60HP Bigfoot

[NealDeal]

Need some help trouble shooting. I bought a boat that had been only used 3 times last year and Zero times this year. I took it out after getting it home and ran 10-12 miles on the river with no problem. 2 weeks later i took it out on Erie and went out 10 miles, then trolled for walleye for 3 1/2 hours at an idle. went to go back in and it ran great for about a mile or so then just bogged down and wouldnt go above an idle. after going 5mph for a mile or so it started to pick up RPM's and then ran about 75% of power the rest of the way in. Got home changed the plugs( they looked wet) thought all would be ok. Went back out on Erie Sunday and boat ran great for about 2 miles then bogged down again. Took plug out on the lake and they looked fine. shut motor off and fished a while, started up and and ran great for about 2oo yards then same thing again. I'm baffled at the inconsistancy of this. ANY IDEAS? THANK YOU IN ADVANCE