The engine will hesitate and stumble only in 4th gear(overdrive ) between 1900 and 2200 rpm, and only if i am accelerating slowly. If i push it a bit it runs fine.
Complete tune up done, new plugs and wires.
I always use Synthetic OIL

Does anyone have the same problem, if so how did you fix it.

98,ZX2,ATX 140000KM

Try cleaning the MAF. Worked for me when mine did that. Exact same RPM range mine did it too. There's a MAF cleaner available but I used electronic cleaner spray and it worked too.

Where is the MAF?

It's under that 4 or 6 wire oval looking right at you on the airbox just past the air filter. It's in the intake stream.

ok thanks... i'll check it out and see if it's dirty and clean it if necessary. I'm also doing other long over-due maintenance. I will be replacing the spark plugs and cables.