94 ZXR issue

I've restored a 94 ZXR750 L & she's registered & good to go but I've had carby problems that are now sorted but I'm left with an electrical problem I think.
She plays up at about 3 to 4000K rpm & we thought it was fuel starve so checked the carbies & found a big problem, so fixed it with another set, but still the same problem.
Does anyone know of any issues common to them like ecu or plug caps, leads or coils. Whatever it is is common to all 4 cylinders cause when she plays up there is nothing, no firing, no backfiring nothing, she doesn't cut out just wont rev. When its on the stand it's perfect, just seems to be under load at low revs, although its good in 1st & 2nd gear ???
Any help will be appreciated.

Sounds like your float bowl vents aren't connected to your airbox/intake, when road speed rises so does the pressure in your airbox (thanks to Kawasaki's ledgendary ram air system) & if that pressure isn't passed onto the float bowls the engine will starve of fuel & the bike will behave exactly as you described.
Rather than walk you through the process, just Google 'ZXR carb vent mod' or similar, there's lots of info on the net.