I hate to be a wet blanket, but unless you are already deeply in love with the Tigersharks, sell them this spring while they are still running fairly decent and pick up a couple of Yamahas, Kawas, or even (ugghhh!) a sea poo. Speeking from experience as a former Tigershark and Polaris PWC owner, never buy a PWC that is no longer in production unless you are a mechanic with lots of spare parts. It is difficult to find anyone around here who will work on Tigersharks (or Polaris), and it is getting more and more difficult to find parts.
My Tigershark did the same thing as yours, running well for about 20 minutes and then bogging down and not running well at all. I had someone look at it and he said, "Well, you can replace this ($400), or that ($500) and you MAY locate the problem. The problem was it was a 15+ year old ski for which parts are no longer made by the manufacturer.
In the spring of 2011 I sold my 1996 Tigershark (Montego Deluxe, I think it was) for $1,000 with a practically brand new trailer. The trailer was worth as much as, if not more than, the ski itself, as the ski bogged down and was not dependable. I sold my 2002 fuel injected Polaris Virage i last year, after it quit working and no mechanic would touch it. It too sold for $1,000 with trailer.
If you want to have fun this summer, go ahead and run the 'Sharks, and hope that you have no problems. But once you have a problem it may eliminate any minimal value the machine with the problem has. Once again, your best bet is to sell those puppies while they are still running and buy something a little newer, made by a company that still makes PWCs and the parts necessary to repair them.
Sorry I'm not more positive. I'm just truthful.
former sea doo owner, will never own one again