Engine Managment Light on

Hello, I wonder if someone has come across this problem before.

At the moment my Engine Management light is on and stays on. Car runs absolutely fine no problem or noises or loss of power. However, under a set of parameters the 'Engine Service Now' warning comes up. Engine still runs fine but Turbo cuts out. If i stop turn off the ignition and turn on again that Engine Service Now warning goes off (but MIL stays on). The only difference to the ride i notice between the ESN being off or on is that the turbo doesn't kick in when the light in on. In every other respect it runs like normal.

However, it doesn't happen randomly and I can almost predict when the ESN message comes up. If i take my foot off the accelerator for more than a couple of seconds then shortly the message pops up. If i cruise, take engine out of gear and take foot OFF accelerator the message will NOT come up. Likewise, the message wont come up when cruise control is on unless i go down a medium to steep hill. Bizarrely the light has only ever come on when my foot HASN'T been on the accelerator.

I've taken it to a couple of garages and no-one can see to work out what it is. I took it to the local official Ford dealer (at £180 an hour!) who said its most likely to be a stretched timing belt (the belt was a couple of months old at the time) but this cannot be correct as if it had then timing would be all over the place. Even I know that a stretched timing belt would be noticeable immediately. Another garage replaced the MAS which made no difference (although that was not a genuine part. I asked them if they fitted a genuine one whether it would make a difference and they said no). One thing they did mention is that it appears to be the air intake system which is causing the issue. The figures being reported by the sensor are slightly higher than the car expects but its not by much at all.

So, the car itself is running really nicely still, no loss of power (when ESN light isn't on) there are no knocks, misfires, rough driving or anyone.

I'm baffled as to where to go from here. I want to keep the car and I want to solve this problem. I have to take the car for its MOT in May and know it would fail with the MIL light on (though I guess OBD could remove it) but I want to get to the bottom of it.

Has anyone come across an issue like this and what it could possibly be?

many thanks for your help.