I too am having this problem.  Are the reinvites being generated by Asterisk?  I have a test environment where making adding two lines of code to asterisk will help the issue.  My only concern is whether the code change creates a performance issue.  I tried asking the question the asterisk-dev list, but did not get a response.  My solution was to insert a usleep(20000) between the ack and the reinvite.  This delays the reinvite by 20 ms. 

The concern I had was is Asterisk fully threaded or does the 20 ms delay delay all processing of sip requests.

Gene