Assuming you already checked in the "Key Mappings" menu ( under the Couatl Settings menu ) you haven't inadvertently assigned those keys to GSX, which would be the only place in which GSX explicitly use a "keyboard" KEY, during all the rest of the program, GSX normally use KEYS, it ONLY use EVENTS and, the only time in which they are in use, is during an EDIT procedure, that is when edit an airplane configuration or a parking spot, or during the Quick Pushback programming.
During this time, and this time only, GSX will momentarily assign to itself the SLEW EVENTS, regardless which KEYS you might have assigned in the Control Settings.
This means, the one and only time in which GSX could possibly take away a key from the simulator is ONLY while one of the editors is active ( Airplane, Parking or Quick Pushback ).
During all the other operations, the one and only keys always used by GSX, are the two you see in the "Key Mappings" menu. Which means, assuming you already checked you haven't explicitly assigned these to the S or P keys, it's simply not possible GSX would interfere with these. I'm fairly sure this is the case, because you also said other keys stopped working, which again is impossible under normal use ( outside the editors ), because the most GSX will use are these TWO keys, not more.
Of course, I checked and I cannot reproduce your problem. All keys are working normally.
