Fuel or ignition ? holds back power at 2800-3100 RPM

Had this problem with my 1.9 gti she tends to hold back power then shoots off this started about 500 miles ago I have looked at the common things plugs,air filter and checked the fuel filter which was changed about 10k ago also the fuel pump was done at the same time. I did notice a occasional clicking at the the fuel pump is this usual? (I know the pump has the buzzing noise which is right). Also visually checking the accumalator this looks as old as the car could this cause the problem?

Car spec :
1900cc stealth racing pistons and fully lightened and balanced, reground and rebored cylinder head with big valves, piper bp270 cam, Audi 5 cylinder throttle body,5 cylinder warm up regulator,16 valve injectors, polished inlet manifold, stainless supersprint 4 branch manifold, custom made free flowing stainless steel exhaust, custom made duct for air box with K & N filter.

Just changed the plugs and fuel filter marginal better but not 100 %.
I noticed this accured not long after having the car rolling road tuned the tuner did turn back the timing 2 degrees could this cause it ?

Maybe worth rechecking ignition timing using a timing light. Also have a look a the vacuum hoses on the dizzi and throtle body these are nown for spliting at the ends.