Intermittant juddering of car at higher speeds

I have the same intermittant fault of juddering of the whole car usually at higher speeds, loss of power, speedo juddering but all goes away when clutch depressed. Been in several times over last year. Vauxhall garage now been on to HQ and say say problem for my engine number is "Sticking Exhaust Valves" as carbon sticks to them" which I think is strange. Thought it was so like dual mass flyweel problems. they say no flywheel problem on this modeL thats 1.9L engines. Annoyed so much work as 20months old and 18K miles
What should i ensure they replace they seem to be just ordering new exhaust valves.