Thanks everyone for all your wisdom

So now there is the option that the chatter is preignition coming from shot valve stem seals. I am prepared to change the seals and wanted to figure out the reason for the chatter at low revs...because I am there anyhow.
I started to remove the airfilter, removing the plugs. First thing I saw was cylinder 8 was very 'wet', engine oil was dripping down the block coming from the plug and the plug inside was completely covered with sludge and oil. Looking into the hole I saw a lot of engine oil ans oilt soot there. I guess a lot of oil is leaking down the valve guide there at cylinder 8 (which had not great compression when I tested a year ago, 120psi versus 160 at the others) so there is a great chance that the valve guides are shot and killed the valve stem seal.
I'll see when I'll remove the rockers and springs. I think a radial play of 0.5mm is the max. If I have more I should remove the head...which I don't want to at this point.
Can this very oily cylinder with dripping engine oil cause preignition?
I was also wondering why the oil is dripping down the block (and causing these ugly spots in the Garage and driveway). Could this be a loose spark plug?
The spark plug is totally covered with oily black soot and I assume I need new plugs? All the other cylinders are 'dry' on the outside.
Martin