Check that the throttle body boots are securely fixed to the engine and, more likely, that the air box boots are securely attached to the throttle bodies.
The front jubilee clip is an arse to get to and has to be on the right way round to sit against a plastic tab on the air box so you can properly tighten it.
If poorly or incorrectly fitted then the front boot can detach and completely mess up the fuelling under load. Next options would be;
Throttle body balance.
Bad Spark Plug.