Had this problem with old engine. Swapped engine with a Jasper engine due to other issues after 214,000 miles. Still have this problem with the new engine, so issue must be emissions related.
I have two issues that are probably related. First, the cold idle runs at about 2K - 2.5K. Once it warms up AND has been driven a few miles, the idle runs at about 900, creeps up to about 1300, sputters and drops to 900 again. This takes about 10-15 seconds. One more issue is probably best described as a "dead spot" in the accelerator when decelerating. At speeds between 10 - 30mph, when I let off the pedal slowly, it feels like I am completely taking my foot off the pedal quickly (almost like I'm tapping the brake at the same time).
I've triple-checked the coolant and it is fine. I even checked it on an incline. I have checked the throttle position sensor with an ohms meter and factory service manual. Everything is with specifications. I've check vacuum hoses for leaks.
TIA , AG2