PWC don't go full throttle

[firstpeder]

I bought a used PWC last year. This year when I go full throttle it sometimes goes 20 mph then takes off to 40 mph, and then will be jittery and struggle to go fast.
Is there a likely cause or is this too broad. I probably need to take it to for service, but not being handy I want to at least have an idea of possible reason for this to occur.

Thanks for any help you can provide

firstpeder