is vacuum modulator sometimes = P0300???

This is my last thing left to check on my GA before I start any internal surgery work on this engine.
4T60E transmission vacuum modulator, I did not get chance to check that thing with my vacuum gauge, but if someone already knows what happens with the vacuum from the intake manifold when 3.1L is idling in park? Is the vacuum line from the intake manifold blocked by the vacuum modulator when GA is in park? I do not know much about automatic transmissions, so if someone can explain me where are the other lines leading from the vacuum modulator? I see one “T” splice for vacuum line it goes somewhere underneath the battery I see some kind of black canister. Is that canister there to collect vapors from the transmission or? The reason I am asking this is that my p0300 continues to show as the pending code. (no MIL). And only when engine is in P or N. Is it possible that vacuum modulator is allowing more un-metered air to come in and to cause rough idle and random misfire mostly on cyl#3 and sometimes on cyl#4. When the car is in D or 1,2 and 3 all cylinders show 0 misfire on my scanner. Car has a lot of power, sometimes on WOT I can feel late up shifting, but it is not that bad to assume that my modulator is bad or maybe I am wrong. Also I do not see any ATF in the vacuum line from vacuum modulator, and vacuum modulator does not leak ATF from the side where vacuum line connects, but it leaks ATF from the side where it goes into the transmission case so I think it is OK as long as I do not see ATF on the vacuum side. Did anyone have any vacuum problems with that thing?
And finally some snow for Bears J

If you are still getting misfire codes, I'd say its your injectors or a faulty FPR.

Either replace or clean your injectors.
The whole P-N thing could be your PN saftey switch flasking out.