Anyone with any ideas to this problem?

PPP my 53STI early in July.
A few days after installation my car would cut out when I was driving at low speeds or coming to a stop at a junction. I would only notice this with the loss of the brakes/steering aids. No noise/no warning lights.
Happened 4 times in 5 days.

Took it back to the garage and they said it was more than likely the MAF sensor - replaced it under warranty.
I have just come back from Holiday and its cutting out again? Spoken to the garage and they now think it maybe a faulty ECU. This has only happened after PPP was installed?

Any ideas?
Ta
Aaron