Semi-annual clog at Pittsburgh style sewer vent

[Garry]

My house was switched from septic tank to sewer system several years ago. Ever since, like clockwork, the system clogs spring and fall. The clog can be released by running a snake down the vent pipe located just outside the house. When the snake hits the bottom of the vent pipe, I lift and drop it several times until I hear the whooshing sound. The water can then be seen flowing when looking down the clean out pipe locate 1 foot beyond the vent pipe. Oddly, a strong suction can then be felt at the clean out, but not at the vent.

How is the Pittsburgh(Philadelphia) style vent supposed to be connected (diagram)? Any suggestions or explanations for the clogging?

 

[Bobby the builder]

You may have an edge between a fitting and the pipe and that edge will allow some wast to cling and it, then the solids build and it may take six months for it to need to be unclogged?