We had very similar symptoms for quite some time, however our delay was exactly 15 minutes. We use 2 different carriers, and saw this problem only on devices from one of them. I had a core goup of about 10 users that always experienced the issue and up to 10 others that would fall in and out of the condition.
I worked with enginerrs from both the carrier and RIM for nearly 3 months, and could get no explaination or solution from either.
We finally resolved the issue by moving all of the affected users to another service carrier. I suspect the original carrier had oversold bandwidth and was somehow throttling the connections...