Problem in 5th gear on 2001 Cavvy

Only happens in 5th gear, i will hit the gas and nothing will happen, like i got 0 hp, clutch is fine, never had the problem in any other gear. It happens on and off, sometimes i have power and sometimes i will drop mph on a straightaway without letting off the gas....seems maybe a fuel problem or so....but if it is, why isnt it happening in all gears? Im baffled by this one, would appreciate any help....TY in advance.