My first check would be the HT side of the ignition.  Look and listen at idle for any cracking or blue flashes.  If you haven't already done so, give the spark plugs a good clean as well, or replace them if they look worn (I know it's simple and obvious, but worn or dirty spark plugs cause so many problems on modern cars, but never get checked because people assume it's something complex at fault). 

Then I'd go for the injectors.  After a few years deposits from impurities in the petrol can build up and make the spray pattern uneven and the amount of fuel delivered very unbalanced between cylinders.  Things can get particularly gummed up if the car is run on low detergent fuel (as allegedly sold in some supermarkets - I don't want to get sued!), especially for lots of short, shopping trolley trips.  As you take your foot off the throttle pedal, injection stops (overrun fuel cut off).  Injection starts again as engine speed drops to a fast idle speed, then if necessary the idle speed control (ISC) takes over to keep the engine running smoothly at idle speed.  This frequently happens even in the brief period between gear changes, or as you dip the clutch as the car rolls to a stop.  It works well when everything's clean and free, but with sticky/dirty injectors the engine doesn't get as much fuel as it needs when injection re-starts.  As a result, it can falter and the ISC can't react quickly enough to keep it running, so it cuts out.  The ISC takes a fraction of a second to react and open the air valve.  You don't notice it with an engine in good condition, but when things get worn or dirty the engine can easily die before the ISC reacts. 

You can use an injector cleaner, but the cheapest and most effective way I've found is to fill up twice with Shell V Power or BP Ultimate or any of the other advanced fuels.  In spite of what you may have read or seen in the consumer press about them, the advanced fuels do contain a top quality detergent pack among other fancy additives and two tankfuls will clean a lot of the gunk off the injectors and restore the balance and spray pattern.  (NOTE - I'm not talking about simply a high octane fuel, you need the extra detergent pack that the advanced fuels contain).   The extra cost is about a tenner for the 2 tanks and it's the simplest and cheapest way to find out if the injectors were the problem.  If it makes no difference, you've cleaned the engine anyway and it certainly won't do any harm.  One other thing to look at on the injectors is a resistance check.  Use a decent multimeter, take each injector plug off in turn and measure the resistance between the two pins on each injector.  The important thing is that they are all the same resistance - if one is noticeably higher, that is your problem and the injector needs to be replaced (cheapest 2nd hand from one of the online brokers such as 1stchoicespares or breakeryard.com)

Hope that helps

Tim