Well, not having a fast Quad-core is going to be an issue with AVCHD.
Also, tell us about your HDD setup, their speed, controller type and how allocated.
I have a 452GB drive with 254 free space. I regualry defrag and I'm not sure of the speed but I know I puchases the hdd about a year ago, replacing the old one. I have intel core 2 6300 @ 1.86. 3.24 of RAM. Will a new computer really make that big of a difference?
You're computer is a but underpowered for working with AVCHD -- but you may be able to get by.
It's hard to tell if your sound cutting in and out is related to a lack of power, though, or a quirk in the program.
Do you see this same issue when you output your video to a standard resolution AVI, WMV or MOV?