sometimes i lose pwer when in 5th gear and the throttle does not respond then all of a sudden the bike will lurch foword and goes back to normal

David Brearley, March 2011

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