Since the problem is occuring every 10-20 minutes I would recommend doing a test where you turn off IPS.  See if the problem continues.

Without looking at logs your most likely cause is one of the following:

1) IPS is stopping traffic

2) Collisions or other line static are causing dropped packets

If you do the IPS test and you still have issues then the next test, if you haven't done this, is to check for packet loss internally.  Run a packet sniffer to determine what type of traffic is the problem.  To rule out inside interferance you should disconnect the internet and then perform a packet trace internally.  Watch the inside traffic and look for collisions or static.

If you do the isolated inside test and have no loss then you have no choice but to turn on as many options as possible for logging on the device at the debug level and go thru the logs to find out where the traffic is being dropped to the outside.  This conbined with a more packet tracing/sniffering will reveal where the problem is.

Now if you can not do the isolated inside test then I would skip to the full debug step.  The key is to identify the point of failure and I wouldn't assume that if the device says the interface remains up and working that the interface is the issue.

Another option to consider is to ask your ISP for the tracking portal information.  Generally ISP providers will give business class customers a portal login to view their bandwith useage.  One final option is that the ISP has a problem or that you are utalizing your bandwith and capping out.

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