Disco 2001, 51k, full tune-up, in the last 2000

miles I drained the transmission fluids three

times, added one bottle of Lucas, and changed the

filter. For two months, there were no prolems.

During the last four days, the following symptoms

appeared:

1. cold start: the idle is fine, about 1200

2. warm start: sometimes the idle goes up and down between
700 and 3000 (I cleaned the idle speed control
motor and the trottle body, didn't help.)

3. sometimes the car stalls at the red light

4. on the highway the transmission jerks on any
speed (on D mode, D3 mode, and Sport mode)

5. on the highway, the idle goes up to 3-4k RPM,
but transmission doesn't feel like it's shifting,
at least not right away.

6. the car can drive up to 50mph and remain at said
speed without the application of the gas pedal
(I disconnected the cruise control which didn't help.)

Can this be because of the trottle position sensor,
idle speed control motor, transmission, or something else?

All suggestions are appreciated! Thank you.

 

2-3 check for vacuum leaks.
4, What do you mean the tranny jerks.
Have you had any fault codes? What plugs and wires did you install?

 

I'm fairly sure there is no vacuum leak. By tranny jerks I meant that it feels as though the transmission clutch locks and releases. There are no codes.
The installed plug is:

SPARK PLUG - DOUBLE PLATINUM
Item: NLP100320G Genuine Part from Atlantic British.

The wires are still original. They're 100% in good condition, the car has only 51k miles.