MK4 2L GTI - low revs, cuts out, emissions workshop light

Hi everyone,

I have a 99 vw golf 2l gti which is being very temperamental. Some days it's fine, but others its a complete pain. The revs drop away down when stationary and sometimes it will cut out and the 'emissions workshop' light comes on. It also chugs and suffers from a lack of power at times. The revs seem to bounce up and down slightly from time to time too.
The car doesn't have a catalytic converter...is this the problem or is it something related to air/oil filter or fuel injectors?

Thanks for your help.