Youre not in too high of a gear are ya,when it begins doing this? The gear you choose to be in,vs your speed,during deceleration will either help you slow down normally (via the engine brake effect) or it'll cause the car to buck,lunge,etc.So being in the wrong gear is the first likely cause.Your iac valve is supposed to be opened fully to prevent stalling,upon decelerating,because of the throttle blade being slammed shut quickly.If its not doing so,that could be another cause for the bucking.
The iac valve and VSS (vehicle speed sensor) both come into play when decelerating & when coming to a stop.
The vss comes more into play when the clutch pedal is down though.
The following is only relevant if the problem occurs with the pedal down or the shifter in neutral.
If the clutch pedal is down or the shifter is placed in neutral,when decelerating & coming to a stop,idle will be affected by input from the CSS (clutch safety switch) & NGS (neutral gear switch) The ECU uses these inputs to control idle,so an issue with either can affect idle.Theyre not relevant to you though since you had the car in gear with the pedal up.
I would try pulling codes to see if the condition has set any codes yet.A jumper wire and your MIL is all you need,if you dont own a scanner.
Connect the jumper wire between the STI terminal and the STO terminal at the diagnostic port under the hood then turn the key on/engine off and the test will begin. Make sure the engine is at operating temp,the shifter is in neutral,the ac is turned off and the clutch pedal is held down during the tests.Otherwise you'll get codes for the ect sensor & code 67.The link below lists details on the procedure. EEC-IV