I finished my Tigra A recently (built using a lot of parts removed from my Corsa sport a few years ago)

The issue I'm having is with the VX performance inlet manifold I've fitted. I ran it on the corsa for years and it was OK. The only thing that used to do my head in was the EML coming on on motorways and the engine feeling a bit 'flat' sometimes. I always knew that this was down to the blanking plate fitted to the inlet manifold (where the throttle body mounting bracket originally fitted as this also allowed the exhaust gas to recirculate)

I found a fitting guide for the Mantzel Powerbox in an old magazine. It used a metal pipe from an adapted plate to run from the inlet manifold to the powerbox (to allow the exhaust gas to recirculate) This was included with the powerbox kit. So using the images and descriptions I made an exact copy of the plates/pipes and fittings. I fitted it feeling pretty confident it would work, took the car down the road but almost straight away I noticed a lot of hesitation at low revs and at constant throttle. When accelerating hard it pulls really well but generally is pretty awful to drive. The EML is not on despite the issues. I've done data list checks on Tech 2 and everything seems fine. I've been over all the other work I've done and everything is OK - no air leaks - all pipework is correct ETC. EGR valve is new (Genuine Vauxhall part)

Has anyone ever come across this before? Any information or suggestions would be very appreciated!

Phil