Electric window keeps intermittently not working - burning smell with it

Passenger side electric window keeps ceasing to work - sometimes leaving window open. Seems to co-incide with an acrid electrical (?) burning smell that fills the car - originates from under the bonnet somewhere. This was intermittent but since it jammed with the window open for 3 days I have been a little loath to use it. Just out of interest I still get the burning smell on the odd occasion even though I'm not using the window... Ideas??