My 850 GLT 2.5L 10V Manual has developed a fault when applying power or decelerating. It is especially noticable at low speeds. When depressing the accelerator to go faster or taking your foot off to decelerate the car jerks (single jerk)as it takes up speed or slows down. The fault also makes gear change unpleasant and difficult to do smoothly. It's as if the engine revs are incorrect for the speed being travelled. The car sometimes struggles to maintain constant revs at slow speeds (20-30mph) The engine also sometimes over-runs after turning it off .

I've had the distributor cap, rotor arm, HT leads and coil changed and have had the throttle control unit cleaned out, all to no avail.

I was begining to put it down to wear and tear and old age but strangely, for two days recently, the car suddenly drove perfectly and was back to its old self and as smooth as silk but the fault returned on the third day. Is it a ECU problem? No fault codes show up.