Vauxhall Vectra loss of power

Over the past few months, i have bee experiencing some problems with my car. At first it started with the car struggling when idling. The rev's would drop and then try to compensate, the revs would drop to around 700 rpm then raise to 1000/1100, almost pulsing. Then the car started to lose power now and again when driving, this is only occaisionally. this seemed to sort itself out and never stopped the car, it just seemed to slow down for a few minutes. This has got worse, now the car will slow down and i'll have to stop. After stopping the engine it will run again fine as if there was no problem. Them more recently the car did the same, but upon restart there was a plume of smoke for around 1 sec out of the exhaust then a smell of rotten egg. The car has cut out since but without the smell.
It seems there is no particular time when this happens, it doesn;'t seem to be direclty related to temperature nor the length of time the car has been running.
I thought it may be the cat but would this result in loss of power for a minute

Hope someone can help. If you do have an idea, or have had the same problem, how much does it cost to fix?

Thanks very much

Gav