lexus-is2

Ok so i am having some random issues where 2006 IS250 every now and then decides to become sluggish in throttle response, if i turn the engine off and start it again it all returns to normal.

I changed the MAF sensor and the Throttle Body without any success. One pattern I have noticed is that I have a Joe Z intake with modified Airbox for the nice intake Growl. When this problem occurs, the intake noise completely disappears and cannot be heared at all even way over 5000rpm. It almosts sounds like stock revving again until i restart the car and it becomes fine.

My question is, what kind of sensors or things would cause my Joe Z intake noise to completely stop and then upon restart it comes back? I thought maybe a vaccum leak of some kind, but then a vaccum leak the problem would occur almost all the time, whereas my problem occurs very randomly every so often, it's very hard to replicate it.

I noticed a vacum switching valve near the throttle body, just wondering what this does?

Any helps guys? would really appreciate some suggestions.

Thanks