I had similar with mine except it didnt stall but revs went well low.
Interpro thornbury have been working on mine as the car was also running lean when put on boost. the answer is my standard ecu is knackered and at the mo they are fitting with the latest OMEX system, quite costly though, but will be worth it, should have it back next week. if it resolves my probs will let u know

rgds nik