98 honda accord hesitates

i changed the engine about 6 months ago, its a 1998 honda accord lx vtec, the engine was gone so ithe mechanic put in a ulev vtec. as of late when i drive off, after about 10 seconds i am pressing the gas pedal and the car is not exilarating, then after a few seconds its back to normal. the funny thing is that when it is hesitating and i am flooring the gas pedal it is not even reving up. i have taken it to several mechanics and the cant find the problem, could anyone tell me what they think is the problem, i even changed the gas pump but the problem is still there. Thanks