Hi, have a strange one!

I have a mk5 rs2000 with standard i4 engine.

The car will start straight away, idle and rev all day without a problem.

When I put in gear and start to move away however, it runs for about 3-4 seconds, then stalls for about 3-4 seconds, then fired back into life for 3-4 seconds, etc

Fuel is new, tried with idle control valve disconnected, checked vacuum hoses and can’t figure it out.

Would anyone be able to point me in the direction of what to try next or has heard of a similar issue?

thanks in advance!