[DannoXYZ]

Most likely a vacuum leak between AFM and intake valves. Driving around in non-boost conditions, leak sucks in extra air that's not measured by AFM. As result, DME doesn't inject enough fuel and you get stumbling.

Under boost, the situation is reversed. Air that's measured by AFM is blown out of the leak by boost pressure. However, DME still injects sufficient fuel to match original air volume. Results in too much fuel, but this doesn't cause stumbling.

Common sources of leaks are:

- AFM clamp to J-boot
- J-boot clamp to turbo
- torn bypass valve disphragm
- short intercooler hoses, they get cut by too-tight hose-clamps
- conical hose going into throttle-body
- idle-valve hoses
- any tiny vacuum hose coming off intake-manifold
- intake manifold gaskets
- etc.

To fix, remove everything between AFM and intake valves and inspect for cracks and leak. Re-assemble with all new hoses and gaskets.