Did you replace any upstream sensors? As I stated previously these can go out of normal operating spec, not enough to throw an error code directly relating to them, buy enough to alter the mixture so that the downstream doesn't behave as the ECU expects.
A faulty cat should be easy to diagnose, just get an emissions test done.
Upstream sensors should switch fast, downstream sensors should switch slow - this is a correct operation. If downstream start switching as fast as the upstream then the cat could be faulty.
Having said all that does the IS250 use an O2 sensor upstream? I thought it used air/fuel ratio wideband sensors - if it does use A/F sensors then your graph is incorrect and your software not set correctly.