Renaught Scenic Megane?

Its an R Reg vehicle with 2.00 petrol engine. For nearly six months i have taken my car to reliable local garages. Both have done diagnostic tests with no results. Changed spark plugs, service and additives to fuel tank. The problem persists and getting much worse. The car starts up OK, then has a strong judder , worse when really slowed in traffic, pulling away or stationary. Car is fine 50 -80 mph. The other problem connected or unconnected is that it rolls backwards and will not engage in any gear 3-1 or D when it is left stationary for more than few minutes. The longer it is parked, or angle of slope increases the time from 1min-4mins to suddenly jerk into gear and drive forwards. Does anyone have any ideas to remedy these faults as the car is getting undriveable.