Good luck! 
I fitted a new pipe to that recently and although it was clear (could blow through it) because it was too small a diameter it caused problems like you have!
Do you mean the replacement pipe you bought was of a thinner internal diameter and caused problems, as the one i've bought and fitted from GSF looks to be slightly smaller, and this morning i've still got the same issues

Saying that the connectors at the ECU and throttle body cant be more than 2mm in diameter?? Runnin out of ideas..............
I replaced the relays so we count them out.
Update, car wouldnt start, had AA out , the coils nackered, he plugged on in that he had and the car was running smoothly, anyone know were i can get one from over the bank holiday? ECP are supposed top open sundays in wembley but there not picking up, and I dont think GSF open?
Updated new coil has fixed the car, its revs cleanly round to 7k, no misfires & only 26 quid from local motor spares shop

cheers