My wife has a 2004 A4 Diesel 3.0 TDi Quatro. From time to time, the Glow Plug Warnign light comes on. When the light is on, the engine hesitates when she tried to accelerate? Any idea what this means? To I just need to replace the spark plugs? If so, can an average guy do it himself or do you suggest a garage? Any suggestions on where to but the plugs and any advice on how to replace the plugs? Thanks in advance!