Hey I had been driving my 94' Volvo 850T everyday for over a month without a problem and GREAT gas mileage 20/27ish til few days ago two problems popped out on me

when I put it in reverse the motor would shudder real hard like Bang!! Making motor cutoff sometimes, it even cut off while driving all day then drive at 10 mph slowing down and the motor just shutoff by itself with all lights on the dash but always start right up with no problem.. All fuel pump, both relays, filter are replaced, along plugs and wireset too, replaced all injectors with used one i even have trans completely flushed with valvoline dex/merc, all I can think of what may be wrong are neither coil or cam sensor is bad, maybe idle motor? any ideas?