6N2 starting problems

Hello,

I have a 1.4 16v 6N2. Very occasionally when i go to start it the engine will turn over but wont actually fire. When it does it fires up for a few seconds before dying. To get it started i have to turn the key the engine will turn over for about 5 seconds before it fires i then have to rev it really hard (3000rpm) to keep it alive. After about 20 seconds it is good to go.

As you can imagine this is not ideal, like I said it is quite rare but still annoying none the less. Any ideas of what is wrong will be much appreciated.

Thanks in advance,

Sam