Keeps losing power struggling to drive over twenty mph

First all I have had this car from new and had nothing but problems! (Never recommend)
I was driving along on the m25 and the engine went. The garage came and picked it up. They put a new engine in and said they had to change a couple of sensors on it plus I got it fully valeted costing me £890 I picked it up and not even five miles down the road. Struggled to go over twenty mph and a light came up with a spanner and a car. I stopped at the shop called the man who fixed it back up told him. He then told me to bring it back. I then turned on the engine and it was like it reprogrammed its self and was working fine again for a couple of miles then did the same thing again. When I got to the garage he put it on the machine and said it was the air flow system. I then waited for the part costing me £120 I paid that, then I was driving off all happy and penniless again and then it started again. Called him up yet again told me to take it down there again! He changed the flitters and plugs, but still not working and still losing power. He and another two mechanics have looked over it and they still can't find what is wrong. Can someone please please help I really cannot afford anything more?