Recording from Digitech RP500 sounds broken

Hello, Need some help. Audacity ver 2.0.1 (installed from Synaptic) on Ubuntu 12.10. I have a guitar backing track which I play in audacity and then record over USB using my Digitech RP500 guitar processor. Everything gones on fine but after sometime, a minute or so, the recording instrument sounds broken, kind of stacatto or tremolo like effect. I can’t understand this behaviour. After sometime, it is back to normal. In the preferences for Audacity, under Devices>Recording I have selected “DigiTech RP500: USB Audio (hw: 1,0): Line:0”. Software Playthrough is not checked under recording. Checking it also results in the same behaviour. Under Latency, Audio to buffer is set at 100 ms and Latency Correction is -130 ms…All default values.
What am I doing wrong?

Probably nothing.
When you get a bad bit like that, have a really close look at it (zoom in really close) and see if there are tiny bits missing in the recording. If there are, then the problem is almost certainly the USB. USB was not designed for continuous time critical data streams, and sometimes the computer can be “busy” and put USB on hold briefly, but this wrecks the recording. Probably the best you can do is to get your computer running “lean and mean” so that there are plenty of resources available. Unfortunately the default Ubuntu has become rather fat and sluggish in recent years so you may need to do some tweaking (or change to another distribution). You may find that using “GNOME classic” rather than Unity gives a significant boost to performance.

Thanks Steve for the reply. I’m already running gnome classic rather than the obnoxious unity. I do have 2 Gb of RAM, but guess will have to really get down to system monitoring. Probably for the duration of the recording I will have to free up resources. No idea how, but will figure out. If successful will post. Thanks again, appreciate it.

One other thing, very occasionally I get the problem that my USB sound card just does not run nicely. A reboot fixes it (probably just logging out and in again would do the same).