2004 Citroen Xsara Picasso engine fault indicator

In May 2011 I changed the Citroen radio which disabled a lot of the computer settings, like miles left of petrol, etc. A few weeks later I was on the motorway in the outside lane when the engine fault light came on, and I lost significant power to the engine. I started to slow down, got to about 50mph, when the engine seemed to regain its strength and I speeded up again. The fault light was on the following day, but never came on again and the car has worked fine.

Then the same thing happened yesterday, 15 monhs later, again at a 70+mph speed. The light is still on today but the car seems fine. Googled the problem and seems I could easily fork out for unnecessary garage costs for an undetectable problem.

Any thoughts?

Joanne