Tow car problems

unclewal

I have a 2004/5 Nissan Xtrail which has developed a problem of power loss when under strain ie slight incline rising. It seems to occur at about 2500/3000 revolutions and can occur in 3rd and 4th gear during acceleration. A release and reapplication of pressure
on the accelerator seems to help. Garage mechanics not too helpful so far, any idea's.