Full load is where you press the accelerator down to the point where the car won't pull any harder.

To test the exhaust, with the engine running, get someone to block the tailpipe end with a rag, while you stand over the engine bay. If you hear a hissing near the engine bay-there's a leak! A leak near the back of the exhaust isn't important.

Failing coil packs often manifest themselves at about the 2000-3000RPM range under heavy load.

UPDATE

Autoelectrician was at a loss about my car.

Apparently this model has two cats, one in the manifold and one on the outside. He examined the physical condition of both and his opinion was that (looking at the bits he can see), they don't need replacing.

I asked about oxygen sensors but he said it's unlikely as the only error code that shows is the cat not performing properly. He said there's a very small leak in the exhaust but that's near the tailpipe so it wouldnt effect the cat.

He's switched off light and I got the price of a cat (genuine part) is around £500 - and thats for one, and presumably would have to replace two!!

I've monitored the running of my car.

The only thing I'm noticing is this strange "lumpiness" when in 3rd gear, around 25mph. Once I get into 4th and driving up to 30mph, car runs OK.

The car idles OK. It also runs fine from 0 - 20 mph. It's just when the revs hit around 2000rpm and the speed is over 20mph but under 30mph that the car feels like it's jerking a bit.

Don't want to really get rid of the car, it's got 10 months mot and is in lovely condition but I'm dreading the light coming back on. Although saying that, whatever problem was there before causing it to come on is still there as haven't had any repair work done on it.

Why does the light take a few weeks to come back on if there's a fault there all the time??

Was going to ask my mechanic to do a few small jobs on it just to try and eliminate it that way possibly. The following ideas are from info I've googled - could anyone else suggest if they are worth doing or not. If anyone thinks they could solve the problem with my car, I would bring it to them - I'm near Chester.

Here's what I'm thinking of doing:

Cleaning throttle body.

Cleaning breather hose.

Checking spark plug settings - new ones were put in when the car had a major service when I bought it.

Ignition coil.

Can't think of other things I could try which aren't too expensive. It's quite subtle when it does the jumpy thing - my partner has driven it and he didn't notice, but it's definetely there and always around the same point.

Faint buzzing sound is probably the fuel pump.

Does it sometimes make the noise briefly when you unlock/open the drivers door? Many cars now automatically fire up the fuel pump when you open the door in readiness for starting.

Probably too late to contribute meaningfully but I have had a similar problem for months - finally diagnosed as the coil pack by my 'round the corner' garage. No warning light, jsut juddering at 22mph/3000 + revs, and two weeks at the VW dealer while they investigated the driveshaft/gear box/engine cylinders. Luckily only got charged for a fuel pump when they asked me to take it away!