OK, I'm going to contradict everyone on this A/C and surge problem, because mine was fixed.
I took my 98.5 Mystique in just before end of warranty on this very issue. The surge caused the revs to jump by about 250 RPM every five or so seconds and caused uncontrolled acceleration when it happened if I was at slow speeds (pulling up at a stop light). Over the course of six (!!!) weeks my dealer recharged the system and replaced everything.
The "replace everything" was prompted by a hard-to-find leak in the refrigerant starting during the first repair (recharge and replace accumulator). Over the six weeks of trying to fix the leak the following was replaced: all hoses, o-rings, accumulator, evap core, and compressor.
The two pluses from this are that I got to drive an Escape for most of the time for free and that the surge is completely gone. OK, there is still a very slight hiccup every once in a while at a stop light when A/C is on, but no surge.
So it is fixable, but I wouldn't want anyone to go through what I did.
Oh yeah, the leak turned out to be in a pressue switch behind some electronics, which was why it was hard to find - no-one looked inside the switch.
