Has anyone else seen this problem or is it unique to my particular combination of hardware/software? I realise I'm a little out in the fringes with my set up.

PROBLEM:
When converting format about 5% of conversions get into infinite loops and never finish. This has always happened and can usually be traced to strange combinations of characters, multi-column text intermixed with graphics, etc. No problem.

Until recently this did not noticeably affect computer performance. I could go about other activities and when I noticed the conversion had not finished I could just open the conversion list, stop the bad job and deal with the bad file. Now when Calibre enters this condition my computer slows down to a crawl taking 30 - 60 seconds to respond to a mouse click and longer to open a new window.

Killing the job and closing Calibre DOES NOT correct the problem.

After closing Calibre the system monitor shows an apparent runaway process:

calibre-parallel uninterruptable

CPU usage 20-28%, both core
Memory usage 100% (2 GB total RAM)
Swap 50% (4 GB swap partition)

Killing the process restores about 90% of performance but the monitor still shows:

CPU usage 50-60%
Memory usage 30%
Swap 10%

with only normal background processes running although I'm not familiar enough with linux to identify them all.

Restarting the computer restores things to normal.

I've used Calibre extensively in the past but not much the last few months during which I did a few upgrades that might have affected the situation.

Current setup:

AMD Athlon II X2 with AMD 880G chipset
RAM: 2GB DDR3
SWAP: 4 GB

Ubuntu 11.04 (Classic Gnome desktop, NOT UNITY) AMD 64 version

Calibre 0.8.40 (5935 documents in library)
I have seen this problem when converting text, pdf, rtf, chm to epub.
Those are pretty much the only conversions I do.