I remembered I had this problem too about 4 years ago, but not all the time like you

Changed everything possible AMAL-valve/plugs/leads

Checked everything electrical with the original Ford Workshop Manual and still nothing

Checked fuel pump feed/pressure/flow/fuelfilter

I eventually took it to the Ford dealer BIGGEST MISTAKE EVER!!!

Then I took it to a local motorsports company and he found it in 15 minutes

Problem: cracked MAP sensor inside the ECU!

Changed ECU et voila: problem solved

Annti,
Connect the AMAL-valve back up

Switch on the ignition

If you blow softly in the vacuumhose to your ECU, does the AMAL valve begin to click/rattle
If your AMAL-valve is blown/not there check if you get a pulse signal with a multimeter

If you don't get a signal or not a regular signal try changing the ECU.

Blown MAP sensorsare common (so they told me)NOT THE FORD DEALER BTW

Hopes this helps