droping rpm

i have an 01civic i put a magnaflow exhuast on nw when my rpms are droping down the get to 1500 then drop instently to 1000 then shoot back up to 1400 is this normal:confused:

You put just the tail pipe or a complete system? disconnect the negative battery terminal for few seconds (10-30). then start it and let it idle for 5 minutes.