loss of power

[rainman]

...not that I had much to lose but this seems to only happen in third gear when going up hills, maybe because that's the only time the engine's under any real strain. It's hard to describe but it's like something's holding it back, the power (or at least the speed anyway) drops down for a second and then picks up for a second and so on in a regular pattern until I get to the top of the hill and change into fourth. Does that make sense to anyone? If you think you might know what the problem could be, I'd be grateful if you cold try to explain it in idiot's terms - thanks.

[kevtherev]

...and the engine is?

[rainman]

soz, it's 2L air cooled CU, twin solex carbs and electronic ignition.