loss of power?

Hi
I've got a Nissan primera 1.8 (53 plate) and a couple of months ago I noticed that when indicating (especially left) i was loosing power. Now I have an intermittent fault where I seem to loose most of my power so it takes forever to get up to a decent speed and won't go above 60mph. Sometimes braking or turning the right indicator on solves it. It seems to be worse in damp conditions. I do alot of motoway driving so can't be putting up with this all winter - any ideas?