check your sub zero start resovior, make sure it has fluid. it's in the back left of the engine bay, 90/10 coolant mix. The sub zero start system only work when its cold outside.
The ac stumble, Definately probably the BAC vavle. get a FSM(@ rx7city.com), and do the BAC valve check.

 

90rx7convertable.. your description of stall at idle under load (AC on) is textbook BAC problem. Molotovman is correct perform a BAC valve check per FSM. If it is in specs all it may need is a cleaning. A DIY is here: http://www.1300cc.com/howto/how2/bac.htm

Good luck