Torque Convertor? Idle Issue? Stepper Motor? All?

Hi …new member here. 96 XJS. While driving if I get to about 50 mph and take my foot off the accelerator the car slows down but when it reaches about 35 mph and 1500 rpm the car rpms go up and down between 1400 and 1600 rpm’s while in gear. If I do the same thing and put the car in neutral the car will decelerate to 0 mph and tach follows along. I was told this indicates the torque convertor and to keep changing fluid until clear…then drop pan and change it all. My friend (mechanic) has been working on the car for me and said this does not happen if the car is in 3 speed position ( no overdrive). He also said the check engine just came on and said idle issue. When I bought the car the seller said he thought it was the idle stepper motor.
Anyway to diagnose would be a huge help.

A quick, cheap first step : Your car has an OBD II connector. This means you can go down to Auto Zone, Advance Auto Parts, O’Reilly’s, etc., and get a free diagnostic readout. Or, you can buy an OBD II diagnostic tool which will give you the basic generic codes for the engine and transmission.

OBD2 tools start at about $15. I use a basic u480 model.

I also have a wireless ELM27 adapter so I can use an iPhone (remember to get the WiFi ELM27 if you plan on using an iPhone, Android can use the Bluetooth ELM27) Not much more expensive than the u480.

Every car owner with an OBD2 port should own at least one. (that means if you have a car from 1995 and on, you should have one. It will help you as much in the 96 Jag as it will in your 2021 Ferrari)

I don’t know the car but it will not be the torque converter, it is the overrun shutoff and in my case it happens when a throttle switch is misadjusted.
OBD2 means either something at the throttle potentiometer or the idle air valve, or something related to it? Especially if it says idle issue.
Scanner is good to have.

Actually mechanic said check engine light code came up and it stated Idle Speed Control Valve. He then said he found a hose off and the light is now out however the problem remains and light will probably come back on.
Heres the thing. I’d like to know if there is a way to test it. They are on ebay for $15 and up to $195 for Lucas. I don’t mind spending the $ for Lucas if I know thats the problem. Is it worth buying the cheapo to see if it helps before doing so? Or can I somehow test mine.