loss of power when in fifth gear?

Hi, you don't say what bike it is??? anyway, depends what the engine revs are doing at that moment when you say it doesn't respond, has the bike got a tacho so you can see what the revs are doing? if the revs are climbing when you say it loses power it means a slipping clutch therefore when you ease off the throttle the clutch will re-engage drive suddenly, hence the 'lurch' as you describe.
Otherwise it could be (high-speed??) fuel starvation possibly due to blocked filler cap or restricted fuel tap or carb jets.

Ted
March 2011