Ford Repair: escort 1.9 eng surges at idle, intake manifold gasket, air control valve

Question
1988 ford escort 1.9 lv eec 124000 miles surges at idle and then the rpms drops and the engine almost dies only to repeat the same.  driving the car the idle remains high as if the car is overrevving however i am not stepping on the gas.  initially i replaced the tps sensor the only thing that did was make the eratic revs to the pattern discribed that is engine revs high and then almost stalls and then revs high again.  any ideas or clues thanks talipp

Answer
I would suspect that the 'check engine' light has been on, so a code should be stored in the PCM... that would be the place to start.  It will at least tell you what system needs to be diagnosed.
The fast, surging idle could be due a vacuum leak.  The PCV valve, brake booster, or a cracked hose could be the culprit.  A leaking intake manifold gasket could be causing this.  Sometimes, spraying the area with a mist of water will help identify the leak.
The idle air control valve (IAC) could be the fault.  With test equipment it can be monitored and manually run to the closed position.  It may not be faulty, but could be responding to a faulty signal the PCM is getting.
A diagnosis by the experts at your local Ford dealer might be a good investment, as guessing and replacing parts could get very expensive.
Hope this helps...
Clay