T4 Petrol Loss of Power & Splutter

Hey folks. Wonder if you could help? I have a 1992 VW 2.0l Petrol Transporter conversion (Bilbos). Only had it a few months but been back in 4 times to have fixes re: how it 'runs'. It's only got 45000 miles on it but we're having issues with the exhaust popping/spluttering, over-revving/running rich and also a lack of power on very odd occasions when accelerating. VW have had the van in 4 times under-warranty and seemingly tried to 'patch' the issues but not done the whole thing properly in my opinion. I'm looking for a few leaders from you guys to help me in my conversation with both Bilbos and VW so this can be fixed finally. FYI, it has a complete and full VW serv history but had a non-vag exhaust on it when I bought whih has subsequently been semi-replaced with VAG parts. Could it be that? Carb? Fuel Filter? Certainly sounds/feels like the fuel isn't getting to the engine properly. Thanks folks