I have the same exact problem. I too have a very fast download rate and the decoding usually lags behind the currently downloading file. The par2 checking seems to be fine for the first few, but then on or about the third one (I haven't kept track, so I can't say for sure but it was the 3rd one this time), it gets part of the way through the par2 checking and stops and won't find any more files. No more par2 files are added after this. I can not even manually import a par2 file. I am then forced to close alt.binz and reopen it to get it to work again. Also, if I go into the setup and try to make a change, I get an error message (I don't remember what it is off the top of my head) and can't make any changes. After closing and reopening, it works fine. It seems to work fine as long as there are only a couple of downloads going, but it seems to hang up when there are a bunch of downloads going and it can't keep up.
This has been going on since I started using alt.binz and I keep hoping it will be fixed in the next release, but so far it hasn't (I'm using 0.27.0). I was thrilled when I found this program because it added the functionality I was missing from NewsLeecher while still letting me use the NL Super Search, but this bug is very frustrating.
If anybody has any helpful suggestions, please let me know and I will give it a shot. If not, maybe this bug will be fixed in the next version or two.
Thanks,
Daniel