1995 25hp 2 stroke intermittently loses power

[Sirtubblord]

Good day! So I bought a boat a few weeks ago and although it was supposed to be a great running engine it turned out not to be. First, the impeller was bad and I overheated it. Just for a minute but it did get hot. I fixed the impeller and that part worked great. While test running it in the driveway it became apparent that the exhaust cooling plate gaskets were shot and I also noticed that a little drip was coming out of the water jacket on the head so I took them all apart and replaced the exhaust, head and water gaskets. Driveway test showed no leaks. Took it out to the lake and it just didn't have the guts to get the little 14 foot boat on plane. Brought it home and checked the new lower plug I put in and it didn't look like it even fired. Checked for spark and had plenty so I put the old plug back in and it immediately ran much smoother. Took it back out on the lake and it ran like a champ... for a while. I cruised on plane for about 1/2 hour, then trolled for about an hour, then got back up on plane to get back to the launch. About 15 minutes in it acted the same as it did before I put the old plug back in. I can get it to run kinda ok with the choke about halfway out but still not enough to plane. So my million dollar question is what to do next. I assume this is a fuel problem. Now when I run it in the driveway it kinda coughs pretty regularly and if I try to idle it down it just shudders and dies. It starts right up but just isn't right.
Any help or advice will be greatly appreciated!