that doesnt sound to fun. when i shift i do it mid to high rpm because if you do it in the low rpm range it does a sort of knocking sound i figured out. i read somewhere that the optimal shift points for the v are not even in the physical rpm range of the car. when i shift mid to high i get better gas mileage too for some reason. go figure. the stabiliy control sounds like it needs to be looked at and if im not mistaken only by a cadillac dealership or something with knowledge of stabilitrak. sounds like stability is not on at all anyway. when i drive down the highway with stability on it drives straight as an arrow with no wheel give. when i put it in competition mode and it turns the stability off, i get more wheel vibration, pull, and wheel give. i've gotten that same message with the gray screen on the nav but it goes away.
hope that helps. anymore questions i'll try and help since ive had a few problems with the car also.
hopefully your car is still under warranty? if so just take it into cadillac and have them deal with it.
when i bought mine the previous owner messed with it a lot. stability would randomly engage and brake would lock up for a split second. i had the dealer take care of it but i later found out they just permanantly turned stability off (which i was pissed and raised hell about) which like i said sounds like the problem your having, no stability system in it.
