VW Polo - starts, then revs suddenly high, then dies

Hi, I bought a 1996 VW polo 1.9 diesel in March. It's been great, except that since I got it it's had trouble starting. It will always turn over as soon as I start it, but then after about 10 seconds the revs will suddenly double from idling speed and then die completely. You can sometimes 'catch' the revs with the accelerator as they start to fall, but often it dies too quickly to catch. There doesn't seem to be any correlation with hot/cold weather, although it usually only does it when I start the car first thing in the morning, and it seems to be getting worse. Once it's started and died once it's then much harder to get it to turn over when you try and start it again. Any suggestions please, I really need to sort it out!