AX 1.0 1993 Engine Speeds up & down

Can anyone help with this problem, which happens on almost all journeys. Mostly after two to three miles from start. The engine speed drops for a aboiut 2 seconds, then returns to former speed
for 2 seconds. This cycle then repeats two or three times. After that, the car returns to normal. I wonder if it is the ecu (its a Bosch Fuel Injection). The local Citroen dealer checked the car only a few weeks ago, found nothing wrong (according to their diagnosis equipment anyway). The car was revving high at the time, so they fiddled with the idle settings. But the problem has continued, albeit in a different forms.
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