Vauxhall Zafira shuddering, loss of power and all warning lights coming on

This problem happens on a two weekly cycle and started a couple of months ago. I could be driving along quite happily when i ease my foot off the accelerator to slow down the whole car will start shuddering. When i then put my foot on the brake all the warning lights come up including engine management and i lose power, usually on junctions or roundabouts and then the car stalls, which is pretty scary, especially on roundabouts.

I have had two garages look at it and they can find nothing wrong with it. The RAC man has been out twice and again can find nothing wrong with it. It has been hooked up to diagnostics four times with nothing showing up, i have been told that it could be piston rings, possibly throttle housing, maybe the EGR valve and maybe the ECU.

No-one wants to do anything to it until they know what is wrong with it but it never does it to the people checking it and they say it is a lovely car to drive nothing wrong with it. I am a childminder and my car is key to my work so this is really inconveniencing me and causing stress. After five minutes of cooling down if i rev the car like mad it then goes well again for another two weeks. Please help.