Hi ulrikL!
I use PowerDVD as H.264 codec (but I use MP 1.02). Works perfekt on my EeeBox 1012. and yes this box is a perfekt client for MP.
cu
My guess is you aren't recieving your HDTV via DVB-T. The Danish free to air HDTV channel is terrestrial and I haven't heard of anyone getting the powerdvd codec to work here. To the author: I have pretty much identical issues on my HTPC. Let's hope the upcoming release of MP fixes the issue with HDTV via DVB-T and stutter/async