Octavia 1.8 tsi misfire

Hello guys ! I am having this unusual issue with my octavia 2 1.8 tsi. The car has a perfect idle and everything, but when i rev the car and keep it within the range of 1200-2500rpm the car shakes and misfires. But when ever i keep the engine running within those rpm range and clear the codes the misfire goes away and the car doesnt shake anymore. But when i let it idle for few seconds and again keep the revs in 1200-2500 rpm range the misfire comes back. So far i have tried coil packs spark plugs. Nothing seems to improve the car. Tried unplugging the o2 and MAF but no improvement. The car is stock. I had a chat with my tuner he seems to think it is a high pressure fuel pump acting up as there is a long crank problem also when the car is left turned off for few hours. Please help.