running problems

bit of an odd one maybe but it seems each time i go on the motorway and get up above 60mph the car seems to rock back and forth and lose power

it happened ages ago and the carb was replaced along with the fuel tank and all the filters and then it was fine. but its started up again, bit of a pain since i wanted to go away for a few days. it doesnt do it at all around town or even on a long run over to visit some family just when i hit the motorway, anyone got any ideas?

1.8 carb btw

stay on the A roads  

i would but it would take me a week to get to where i need to go

Can you give usa clue on the engine?.

Is it a solid lifter head? What  water temps are you getting on the motorway? Just loosing slight power or intermittant loss of full power? Is it like applying the brakes whilst cruising then you have too use your right foot harder, type of effect?

Just that the type of power loss might point to the problem.

regards