sudden idle surge

I have a 97 intrepid 3.5L and when the car is down shifting and preparing to come to a complete stop the idle revs up to 1200rpm and then clunks and then back down to 700rpm. I have changed the intake manifold gasket( top and bottom) but I am still having the same problem. Any idea's??

I had the same exact problem but with a different car (buick regal) only mine was surging to about 2k rpm. After a lot of troubleshooting I found out it was a bad connection on the spark knock sensor (which the manual and dealer said the car did not have, but it did!). I unplugged it(1 wire) and cleaned the connection and it fixed that surging problem. Mine was located on the firewall side of the engine under the exhaust manifold (real hard to see).