987.1 S with GT3 IPD Plenum/TB ISSUE...Help!

[Bluestraveler1]

I own a used 06 Boxster S, 987.1 with 48K miles that I bought 2 yrs ago. The prior owner had installed an IPD Plenum and TB that is 82mm (GT3) when the original is 68mm. It has run like a charm except last fall I noticed that after the car warmed up, it would not hold steady throttle at stoplights, etc. However the CEL never triggered. I decided to take it to my reputable and very good Indy Porsche mechanic at RennSport Tuning. He put it on the Porsche diagnostic software and discovered that 1 of the 2 potentiometers in the throttle body wasn't responding to changes to acceleration from idle. In other words, a malfunctioning in the TB. I don't know when the GT3 TB was installed so it could have been awhile ago. My mechanic ordered a Bosch OEM replacement and installed it. I also had him change the sparkplugs as it was recommended at 40K. I picked up the car and drove it for 70-80 miles and it is now doing the SAME THING. At cold start up it idles at about 1050 rpm. After running a couple minutes it drops to about 800 rpm and after another minute or so to 650 rpm steady. It seems that after driving 10 or so miles after warm, the idle begins to hunt from 650 up to 1100 or so rhythmically. I have a followup appt scheduled so he can put the new TB on the diagnostic software but not for a couple weeks.
To all the Engine Mod Heads, I'd appreciate any suggestions or ideas you may have. I asked him about MAF sensor when he had it in the shop but he said there were no codes indicating an issue with that part. What else might it be other than some sort of vacuum leak which I assume he already investigated. HELP

[husker boxster]

I've had the IPD plenum and GT3 TB on my CSS for yrs w/o issues. When you were describing your issues, I was thinking vacuum leak. Hopefully JFP will chime in.

[JFP in PA]

I'd start with a vacuum leak.