Have to agree with everything mercman says.
I had a similar problem on my 1993 Mondeo 1.8LX (UK spec). No fault codes were logged and as you can imagine it tracking down the fault was like looking for a needle in a haystack. A very close inspection of the wiring to various sensors revealed that one of the wires to the PAS pressure sensor had broken internally (check for this by feel) and the insulation had started to deteriorate. This had led to the wire making and breaking contact thus giving the ECU false information, leading to surging at idle. Easy to fix (10 minutes with a soldering iron) but difficult to track down.
Rather than instantly blame a particular component as being faulty (even if a fault code readout points to that component) it pays to be systematic when diagnosing tricky problems like these. Thinking through the situation logically, the areas to check, as highlighted by mercman, could all cause surging at idle. Check everything (especially wiring and connectors to all sensors) very closely indeed.
Good luck.
