Must have been unlucky dude, I've got an 89 model with 55k on the clock and it's still sweet as a nut!
I think you just got a bike that was not looked after too well. You should really check out the bike as best you can before you buy. You probably did, but it's just bad look.
I have had lots of Kawasaki's, and the GPZ'S are a fantastic bike. Have a 1991 CBR250RR that I fixed up as a hobby, but my next bike will be a GPZ500s, then the 900R Ninja, and I will always ride Kawasaki's. My father has the 86 900R Ninja and it's such a lovely bike.
I would suggest you buy another one in good condition, and I promise you will never get a bike as nice as the old GPZ / new Ninja.
I have ridden many Kawasakis as well as Hondas and Yamahas. I have to say, the Kawasakis have been the best for me. For the last five years, I have been riding Hondas. I should have stuck with the Kawaskis, as I think they are better in many areas than the Hondas. I am thinking about going back to a Kawasaki. Many Hondas in the past have had engine problems that the equivalent Kawasakis didn't have. Honda aren't all what they are cracked up to be. I have a CG125 for around town, it's okay, but not as good as its rival, the KH125.
Well I know what you're going through. I have bought a 88 GPZ 500 with 35,000 miles on the clock, full Venom exhaust, lovely roar, but I've only had it few days and my battery dead already. Took it out charged and it still died. Is this a common problem with old GPZs, because before buying I looked for reviews and 95 per cent of them were; it is a nice quick 500, cheap to run, and the engine is bullet proof.