Espace MK4 2.2DCI Automatic Nearly Stalling Down Hill

I have a Renault Espace MK4 2003 2.2 DCI (automatic gear box). Car runs well apart from when going down a hill then braking, slowing the car down to around 27 mph in 3rd the engine starts to vibrate as if it wants to stall. Engine never stalls and recovers. The steeper the hill the more pronounced the problem. Fault occurred for the first time after towing Caravan



The car has done 89,000 miles and I'm getting 29 MPG from it.



There is air in the hand priming pump and I can see air bubbles going into the fuel injection pump, or could the vacuum from the brake servo influence the engine revs.



Work done to date is one second hand injector, new glow plug new diesel and air filter. Cleaned EGR. Cleaned Throttle housing. Second hand Vacuum Valve. New engine mount. Clip (computer) not showing any faults.