Xantia engine niggles (2.0 16V)

[rsmith]

Hi all,
I have an intermittent problem with my Xantia (SX 2.0 16V). The problem only occurs for a couple of days every 3 or 4 weeks.
During the two or so days when the problem occurs the idle speed seems to set itself too high.
This might only be to about 1000rpm or it can go up to about 1500 instead of the usual 800 ish.
Also the engine occasionally will hold back (harshly) when accellerating - rev counter drops to zero and car slows sharply for a second.
This second symptom only ever occurs during the couple of days when the idle speed is playing up.
I have had the engine management log checked - it just registers one fault, for incorrect engine speed.
I have been told that this could be the engine speed sensor?
Having looked in my haynes manual I can only find reference to the
throttle potentiometer (which would make sense as a "speed sensor"?).
There also seems to be an "engine sensor" which appears on the GSF price lists.
Are these descriptions all for the same item, or are there several sensors?
Thank you for any help...
Rupert.