RPM Flux When Decelerating Then Steady When Stopped

[97 Ranger STX]

Hello Rangers, happy Columbus Day.

I have a strange RPM issue. I tried to diagnose it myself and got nowhere. Took it to my trusty mechanic and he's out of ideas. I'm trying to avoid a trip to the dealership. Here's the visual:

When parked at idle, RPM is steady. When accelerating, RPM advances normally and is steady when speed is reached. When I take my foot off the gas and the truck starts slowing down, the RPM jumps noticeably as the needle passes through about 1200. It will bounce between 800 and 1200 until I come to a stop then it settles out at just under 1000 and is steady.

It has thrown consistent P0171 and P0174 codes when it starts jumping. They clear and then reappear a couple of days later. I've cleaned the MAF (and have a new one coming from Rock Auto) and throttle body twice. Looked for vacuum leaks and have found none. The IAC, EGR pipe and valve, TPS, have all been recently replaced. Coincidentally, the ABS light started illuminating about the same time. Replaced the speed sensor and valve; didn't fix that issue.

Ideas? Thanks!

 

[Curious Hound]

I wonder about the condition if the cam position sensor drive gear. Do you see any unusual fluctuations in oil pressure? Also check crank position sensor if it has one.

 

[97 Ranger STX]

Hello Eric. No oil fluctuations, Just looked at the both sensor replacement videos and think it's a good idea for a weekend quick fix to replace them. Thanks!