HD Stuttering on my HTPC

aprox. 80% percent of the 720p movies that i 'manage to optain' causes stuttering and fps drops from 24fps to 14fps. the vsync trick did only make the audio track wait for the video track so it continued to be in sync. was courios to see if the movie could play flawlessly on my gaming computer with a oc'ed i7 3.2gHz processor, 6g trichannel ram and a 280GTX.. And it unfortunate did.. so the problem is isolated to be either a software or hardware problem in my htpc

htpc spec

HW:
7300 nvidia
1g ddr2
AMD X2 64bit processor

SW:
Ubuntu 9.04 (x86 - yea i know my processor is 64bit but the xbmc code is still too buggy to use in that arch.)

XBMC newly installed from the repo. regarding the official howto page.

all drivers etc updated from the repo. (incl. nvidia)


So the final question is.. should i spent Moore money next time i build a htpc or should i take some Moore time reading docs? really hope you can help.

[EDIT]
just tested the playback in xine, seems it played the file flawlessly.. any chance that XBMC can be set to play via xine as an external player?

2secs after i posted the problem i saw that i missed the [LINUX] tag... im sorry