Without knowing the history of your blog... updates, file permissions, prior possible sources of entry, vulnerable plugins and or corrupted themes, etc... my advice would be to review any recent changes you may have made, including all of the above items. Examine your source code for anything you don't expect to be there. Perhaps download a backup copy of your files and inspect for anything out of the ordinary, as well as reviewing your database for users or data that should not be there. Review your log files for suspicious activity if possible, and lastly perhaps seek help from someone with experience in resolving these matters. It seems odd that I was not redirected on every visit, nor was I accosted by the JS Downloader on every visit. Your site behaved normally in fact, on several visits. I had hoped to catch the offending script in action again if possible, but I did not. Do some searching on related terms and see if anything rings a bell. It would be my hope that your host could offer some supporting advice, or perhaps someone here can assist you in a more productive direction. Best of luck to you.
[EDIT] a quick google search of sum4count.net had a ton of hits, and it is identified as malicious and a distributor of malware.
Here is a quick whois on the domain.
(shiver)... I don't think I would visit the address (sum4count.net), just to be safe.