security alarm problem

Normally my 93 Maxima will open the door the key is in on the first turn and on the second turn of the key will open all the doors and disable the security system. Lately, it will not open the other doors on the second turn. When it doesn't, if you open the door you put the key in it will set off the alarm and disable the ignition. This is an intermittent problem. What part do I need to replace. Help!!!