Fiesta's and wiring has got to be one of the worst for issues. Cause really random problems.
It will be hard to offer decent advise without being able to look at it with a meter.

The loom you have fitted was a good one I take it?

Have you got all the correct sensors i.e all FRST stuff matched to the OFAB or OFAC system. You cant mis and match parts between the two.

The engine management loom is a FRST type?

Does the map sensor work?
ACT temp sensor work?
Are you getting a decent constant reading from the CPS?
Do all the sensor pin out to the correct place on the ecu plug.

so much more but these are the first questions.