what are some other reasons other than buffer size do drop outs occur.

I have drop outs when I'm pulling up a plugin during playback in Sonar 3. I've adjusted the buffer size from 64 to 256, and it work for awhile. Then I moved it to like350 and I still get... I'm working with like 8 tracks and about 7 plugins. Its really not hindering me much, but I would like to get rid of it.

I run Sonar 3 and this is my comp

E-machine T6520
AMD Athlon(tm) 64 Processor 3400+
2.39GHz 1GB Ram
WinXP

and I send all my wave file to an 80 GB Western Digital Ext Hardrive (USB)

The dropout occured before I sent all the Sonar files to the Ext HD

I also went Cakewalk and read the step by step guide on dropouts I also have my driver mode to (WDM)

also tell me what you think about the e-machine I just picked it up and if some of you think I could have done better for the price then (((let me know ))I paid about 600 for it. I'll be upgrading in a year or so to a more enhanced PC anyway ( for the time being)

Thanks.