e1200 and 3450 htpc no 1080p help

hi there anyone and every one on the toms forums i have a little problem htpc cant play 1080p

specs
cpu=e1200
video=256m 3450 sapphire
motherboard intel 945 gc
2gig 667
1 640 gig seagate
monitor 28 i-inc


plays every thing but 1080p. I thought that the gpu would off load but gpu-z say the load is only 1%
have tried coreavc and vlc player and ffdhow none worked
the thing i cant get is the cpu load never goes above 60% and yet the video stutters.

any help thanks