The red/gray plastic valve is actually a delay valve rather than a check valve. It will allow both boost and vacuum to reach the CBV; it just delays the signal by a second, perhaps to compensate for the long turbo plumbing but I am not sure that is the proper explanation.

Your symptom could be a hole in the intake or in the turbo plumbing or in a smaller hose that is pressurized under boost, that either allows outside air to enter the system without being measured for fuel (resulting in over lean running) or that allows air for which fuel has been metered to escape from the system (resulting in over rich running).

It could also be the CBV. You can easily replace it with a plastic Bosch unit such as that specified for a 2002 VW Jetta 1.8 Turbo. It might be called a purge valve or pressure relief valve or compressor bypass valve or surge valve or something like that. Probably costs about $35. Could also be a split vacuum/boost line to the CBV. Or maybe it is that red gray delay valve. A possible substitute is a gray/black delay valve, but not a gray/white check valve.

And if the car is bucking at all and you have raised the boost too far, maybe you are hitting the fuel cut off. This can happen when the hose to the wastegate actuator below the turbo develops a split.

Good luck. Might be worthwhile to test before buying new parts. Test with a Mityvac vacuum pressure tester or similar device and by removing and inspecting hoses for cracks or holes or splits.

Philip Bradley