Please Help me with my gixxer

Hello, first post here.

I recently purchased a 93 750 GSX-R, but now it's having some problem i can't figure out.

i had only drove it for about 20 km when it died out.
When riding it, i noticed it started losing power and wasn't very willing rev, and one minute after that, it dies out.

I was furious and took it to the mechanic from whom i bought it. This was during the night, so he said he would look at it the following morning.

Then he called me, asking me if i was the one having the problem, because the bike ran just fine, even on the dyno.

I was pleased of course, and took it back and it ran just fine for another 100 km or so. Then today it started the same thing again.
It started not to rev so well, and then died out.

If i wait a few hours and try to start it, it will run, but wont rev and will die out again. And it seems if i wait a few days, it will start and run fine for some hours maybe.

Well, it will rev a bit, but slowly and unstable. Have only tried up to 5k.

I have tried changing the plugs with no avail.
I have also checked the 2 coal pieces wich seemed to be in good shape.

I am completely lost here, and mostly furious because i just recently purchased it for A LOT of money.

Do you guys know a solution to this problem?

Thanks in advance.

Runi