Vent Problem?

[tp01tp012000]

Hi,
I need some help with the toliet on my first floor. On windy days I notice the water in the bowl moving. After it's flushed the water level in the bowl raises then drops about 1". This is causing us a great deal of headaches due reminents being left in the bowl and other factors. My plumber thinks it's a vent problem so he snaked it but it wasn't plugged. In my attic I have a pipe that runs horizontal from a 2nd floor toliet to the main vent pipe. The next step was to add a new vent pipe from this horizontal pipe through my roof which didn't help either. As a test, he added a 6" temporary pipe on top of my vent pipes in hopes by extending it this would reduce any atmosphiric pressure problem. This didn't help either. Any ideas on what could be the cause? I don't think this is related but.. A while back the 2nd floor toliet above this one was leaking so I replaced the wax ring. If I didn't install it properly could this be causing the problem?

[tp01tp012000]

Something I forgot to mention in my original post. All 3 toliets in my house (1 first floor and 2 second floor) will lose water from the bowl if not flushed. The one downstairs loses more than the 2 on the second floor. We have a septic tank which I had pumped once the problem appeared.