finally got to test this properly: fired up my old PC, installed latest version of AH, got exactly the same problem.
switched to a different router, got the same result.
this leaves my network connection as the only possible cause. pingplotter plots look ok to me, is there a way of testing UDP in a similar way?
btw got a squaddie to test it by counting to 20. missed a couple of numbers up to 10, and most of them between 11-20. also noticed kill messages taking a while to appear recently sometimes.