Thanks, I checked the position of the fuel filter (it has a new line) and there was plenty of fuel in it at the time too.
I haven't replaiced the impulse line (but it's not damaged in any way I can detect), I will do this when I put my next parts order in as I also need new fuel/oil caps (or o rings at least) and AV rubbers.
Like I said, it idles fine while I hold it but a couple of seconds after I set it down the revs start to drop. It then gets the wobbles up (starts to shake as it nearly stalls) and then picks back up again. I get the feeling that the way the saw vibrates when it is on the ground has something to do with it.
As it idles fine while I hold it, I guess the throttle plate position is not the issue. I'll have a tweak of the low speed screw as suggested.