I have a 1998 uk classic , when drive in at some times even at low throttle settings power gose off fullly and comes back on not like runnimg out of petrol more of a switch some times at setting off and others at 60 plus mph but not all the time any help would be good not changed any thing always had full decat and hks dump valve

 

What does that ^^^ mean?

At what RPM and boost levels does this occur at?

 

low rmp its done it and some times when just under a bar of boost can be sat on motorway at 60 mph in 4 th gear will do it or when it feels that way out not all times

 

Come on mate, give us a chance. When your 'sat on the motorway at 60' are you accelerating onwards or are you just cruising at 60 in 4th on the motorway for some perverse reason??

It sounds like boost cut or a large miss. Check all the associated boost pipework and connections for leaks and check the plugs, leads and coils for damage, wear and security.