Failed to connect to binary FirefoxBinary(C:\Program Files (x86)\Mozilla Firefox\firefox.exe) on port 7056; process output follows: null

I am using FireFox 18 with Selenium 2.29.0. While running test exception occurs

Failed to connect to binary FirefoxBinary(C:\Program Files (x86)\Mozilla Firefox\firefox.exe) on port 7056; process output follows: null

It seems strange that this error seldom occurs. Let's say I have 20 tests in my class, the "failed to connect" error occurs in 2 tests sometimes 3 and sometimes it doesn't occur.

Can't figure out why is it happening. If this is some how version problem or something else, none of the test case should run.