problems with my 240. help if possible

[joel]

i just recently purchased a 91 240sx (this is my second one so im pretty familiar) and after i picked it up i noticed little problems with it. I noticed that when i brake real hard at high speeds or even if im on the highway and i hit a bump the car sway backs and forth pretty badly. it varies but sometime it wont stop till i slow down. could it be the suspension? i know its not in the brakes. has anyone else had these problems?
also i am going to put an alarm with keyless on it and usually you need an actuator but i was reading somewhere that you dont. does anyone have some first hand on this?
and my diriver window isnt going down. when i hit the switch i can see( with the door panel off) that the motor turns just a little bit then stops both ways. do you think that its my motor that is locked up?
thanks, joel