I like the yamalube... good stuff! 25:1 is probably more than it needs, but is probably not causing the problem.
Intake leak, funky plug or wire, float level set wrong, crud in a jet, restricted air cleaner, leaky head gasket, bad crank seal, low compression from worn rings...
Gosh, really could be a number of things, or a combination all contributing to the problem.
Start with the easiest ones for you to deal with and hope for an improvement.
Then move on to the next test...
Or maybe someone with the exact same setup and problem will come along with a solution.
Probably a good time to review the CNS tuning thread.
Best
rc