Fluctuating / bad idle

[David]

@ David: the hose from the IAC valve Plenum Body which goes goes back into the air intake hose after the MAF sensor?

Not that hose, the other one, rearwards of it and lower, the positive crankcase ventilation valve and hose. It goes from the top left cam cover (the valve itself) just underneath the air trunking, to the plenum right of the tail of the Mustang on the throttle plenum plaque (so between the IAC and the EGR).

Look at the pictures. Here is the hose on eBay at the moment:
https://www.ebay.co.uk/itm/333612093681

... and the PCV valve itself, (a service replacement item at 80k say Ford, 90k say MGR):
https://www.ebay.co.uk/itm/MG-ZT-260...wAAOSwKNpel4p6

Available cheaper from the States of course, under the numbers I quoted in the earlier post.
When mine went bad, I did indeed get lean-mixture error codes after a while, but not straight away.

David