Ford Fiesta car trying to stall when waiting at traffic lights

Noticed the other day that while waiting at lights (handbrake on, no pressure on pedals), that my engine started to sound like it was about to stall. Now this has happened very slightly before and seems to resolve if I rev the accelerator. So this time I did that and it helped a bit but then the lights changed and as I moved away the stalling sensation continued for a little bit and the engine warning light flashed on and off for a few seconds and then it was fine, engine sounded fine and I got home. Now what could this be and do I need to get it looked at as it is a very intermitent problem but it was worse this time? Is it going to cost me a lot to get it sorted, depending on what it is? Cheers.