Ford Fiesta 1.4 CFI engine hesitates when accelerating at low revs

I have a 1988 Ford Fiesta 1.4 CFI as a cheap extra car. The car is in an excellent condition, but when accelerating at low speeds it hesitates. When it reaches about 3.000 rpms it accelerates without any
problems.
I have renewed the following (low cost) components :
- Spark plugs (Bosch FR 7 DCX+) gap set to 1.0 mm according to Haynes manual. - HT-leads replaced - Distributor cap replaced - Air filter replaced
I have asked at the local Ford dealer about the cause. Here it was suggested that it might be a vacuum leak, getting "fake" air to enter the inlet after the injection unit and thereby screwing up the mixture. This explanation sounded quite reasonable, but after inspection I have not been able to locate any leaks. I have also used some "start-spray" to see if the engine would increase it's revs when it sucked it in (this was recommended by the Ford dealer).
I think the reason might be incorrectly set gap on the spark plugs since it seemed like the problem got better when I changed the gap size from 1.1 to 1.0 mm. But I can't see why this should have any influence at low revs - I would expect it to be a problem at high revs if the gap was set incorrectly.
Another thought that crossed my mind is the capacitor for the ignition system. Can it be broken and would it result in a symptom like the one I have?
Any help would be highly appreciated.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

##############
I didn't see "fuel filter" in your "low cost components" list.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

You are right about the fuel filter. I also changed that one and there were no change. Just had a look at the temperature sensor and it measured 75 kohms. The temperature sensor beside the inlet manifold is used in the fuel calculation that the engine computer takes care off.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.