Ford Repair: Shaking, head gaskets, compressor cycles

Question
hey dave,
 i own a 97 ford mustang v6 3.8L.  for sum reason, i was driving my car one day, it started shaking.  when ever im in a low rpm, my whole engine shakes.. especially when at idle, and take off.  its also an automatic car...  ive changed the spark plugs and wires, and it still isnt working correctly. shakes alot.  some people told me my coil cap, or pack (dont exactally know what its called) might be misfiring.  or my pickup coil is out. i dont know what to do. if u can get back to me asap, i will be very greatful...  thanks alot buddy
-Gurj

Also, a little while after the problem started, whenever i turn my air conditioning on, my car drops down to 500 rmp and then back to 1000 rpm and does that repeatedly at idle.  but when its off, its steady at 1000.  i dont know if thats related, but i thought id just tell u.
thanks!

Answer
First you need to check the ignition system.  Your coil pack could be bad and all of the spark plugs aren't firing.  Check the output of each plug wire, one at a time while you crank the engine with the fuel off. You can pull the fuel pump fuse or trip the fuel cut-off switch.  If all are firing, you should check your compression.  It is common for earlier model 3.8L engines for blowing head gaskets.  If you have spark and compression, you might have a bad fuel injector.

It is normal for the idle to surge when the A/C compressor cycles.

I hope this helps,
Dave