If you are sure it is not a warped rotor or motor/tranny mount, and the problem is the engine (you said it does it once, with the idle dipping then rising back up which would indicate the engine), get it scanned at AutoZone or Advance Auto. If an electrical sensor is at fault, a scan should pinpoint the fault, or atleast get you in the ballpark.
Could be simply that the throttle plate is slamming shut too fast on deceleration, causing the engine to want to stall. But the other sensors see this and the computer makes quick adjustments to prevent stalling. Off the top of my head, I'd check the Throttle Position Sensor (and adjust if necessary), the throttle stop screw (typically a screw to prevent the throttle plate from slamming totally shut which is typically preset at the factory with no adjustments necessary), and clean the throttle body (some tiny passages could be obstructed. TB's get dirty. Mine gets cleaned about every 10,000-12,000 miles when I change fuel filter). Also check the Idle Air control Valve (Mazda may have a different name, but it should be on the side of the TB). Actually, the IACV would be the first I'd check. A Haynes manual or Factory Service Manual would have the test procedures using an ohmmeter for the IACV and TPS to check for electrical fault. They'd also have the specs for TPS adjustments, but I would test everything before adjusting the TPS (and especially the throttle stop screw) unless it is totally out of spec. AutoZone's website may also have the repair guide for your car. Worth a check if you don't have access to either of the other manuals. FSM's can typically be bought on ebay for a good price, and are invaluable resources for everything on your car.
Is the condition repeatable? Is there a difference between instant deceleration and a slower easing up on the gas peddle to slow down? Being an auto, do you have a tach? What RPM's does it do it? And how low does it dip before coming back up?
Hope this helps.
Dave
