Popping and bogging before returning to idle

[reincaster]

My 87 won't return to idle correctly! After I let off the throttle in neutral or push in the clutch, it will drop idle to 500-600 rpms for a second or two, bogging, then back to regular rpm. It will also pop one or twice when I let off the throttle quickly. I checked the dashpot, and it's working as it should. Is this a tps problem
? I'm picking up a set of plugs today, as the current ones are black and sooty, but they still have spark. ISC valve works correctly, clicks after turning car off and when tested as tsrm recommends, it passes all tests.

 

[reincaster]

Also, the only code I have is a code 52, but would that affect my car that way? I read somewhere on here that the ecu pulls timing when senses something wrong with the knock sensor, but will that cause the momentary drop in idle? Basically, you rev the car to 3000 and let off the gas, then it drops to 500 and then back up to 1000 (after cold start).