Nissan Micra S: power problems?

I have a Nissan Micra S, X reg. In the last few months I have had a few problems with her temporarily losing power. When driving it feels like she is holding back, but then all of a sudden it feels like something 'unblocks' and she goes shooting forward and then this will keep happening. There is no consistencing with the problem. It can be very dangerous when pulling out of a junction or round-about as she can pull away VERY slowly. Though, like I said, this is completely random. Also, very rarely first thing driving she can some times stall even though my clutch is fully down. Unfortunately I am a student and therefore, do no have much money at all. I have had my AA break down cover come and look at it and they said they don't know what it is but at a guess they would look at the spark plugs. If any body has any ideas what it is and how much it is going to cost to solve, I would be VERY grateful. Thank you