Hello

I have a 1999 Audi A4 2.4l petrol which in the last month has started to act up. At first it was shuddering at about 1800rpm but you could drive through it. I have taken to my local garage but they are unable to diagnose a problem, there atr no lights on the dash and nothing showing on computer. I then took it to a specialist who had it 4 days but again nothing showed up.
However it was running much better even though they hadnt done anything. The shuddering then came back but at 2800rpm.
I am told it could be a coil pack or HT leads breaking done.
Does anybody have any idea what can cause this annoying intermittent shuddering.
Might it be a blocked fuel filter/
Thanks