Logical Logs full, can not be freed!!! Need Help, Urgent!!!

Dear Sir, Our Informix Online 5.0 server's logical logs got full abnormally and frequently, the whole system was then stopped and we can do nothing to continue but restore the whole system and data from Level 0 tape!!! At first, we suspected the "inadequate" logical logs. But, when we add logical logs from 20 to 60, the situation was the same. We then adjusted Long Transaction high-water mark for 80% to 40% and Long Transaction exclusive high-water mark from 90% to 45%, but we can not solve the problem. From Administrator's Guide, we know that a logical log can be freed only when 1) it has been backed up; 2) All records within the log file are associated with closed transactions; 3) The log file does not contain the most recent checkpoint record. We suspected the existence of Long Transaction which was not closed. But how can we deal with them? Are there any ways that we can force the logs cleared? Any of your suggestions will be greatly appreciated! Also, please tell me the email or fax number of Informix headquarters or technique supporter in the US that I can contact. Thanks in advance Xiaohui Shen email: autodept@public2.bta.net.cn