this seems like the same problem im having.
I got the car with 32k but no service history, just mots.
needed a IMRC which i got new, and the O2 sensor, also replaced.
I had a full service done, and had the cam belt changed at a ford dealers.
they noticed a leaking thermostat housing, which i changed myself
Also fitted a k&n induction kit
had a mot done, no problem
i drove 8k over 6 months and then this problem started.
originally the car always started from cold perfectly, but on the motoway at 80, after about 15 mins, would lose power, like i had lifted my foot, and not recover unless i lifted off, the one time i did try keeping my foot down, it throw a dtc, cant remember the code, but said misfire, only ever got that one error. if i slowed to 50 as soon as it started, it would recover, if i tried to keep the speed up it eventually died, but started again 10 mins later.

I changed the the fuel pump, ended up taking the tank out twice, first time i cleaned the tank that was fairly clean, the filter was a bit crappy but not as bad as the donor ( my 1.6 ) pumps filter, i didnt connect the sender properly and had to take it back out again to reconnect that
the 1.6 is still runs fine with the st's pump.

ive also swapped the MAF with the 1.6 and the 1.6 runs fine.
I've checked all the vacuum pipes ( i think ) ive had them collapse on the 1.6 which took me a while to find, but all the st's elbows and pipes are in good condition, looked for bad connections, bad earths, but everything seems good.

The problem has gotten worse, and now sometimes, lately more often than not, it dosnt want to start, i have to hold my foot down, it starts, stutters, maybe a few pops from the exhaust, and eventually the revs start to come up and it gets better after a few mins, this is very random, as sometimes it starts and idles perfectly, although now rarely will it rev smoothly to the limiter.

being curious, i took a look at the vvt solenoid ! which came apart rather than come out, so i had a mechanic remove the stuck half and fit a new one. not cheap !

i had not used the car for almost two years, i took it for a mot, and its failed on emissions! less than 10k after passing with good emission results.
its definitely idling rich

i see a idle control valve test that said to rev at 3k for 10 secs with lots of electrics on, then lift off, if the revs drop below 1000 and it almost dies theirs a idle valve problem, I tried this and it does cause it to almost die, so i wonder could this cause problem at 80 mph after 10 mins.

I bought a cheap bluetooth can bus diag thing, which lasted about 10 mins !!! and burnt out ( ive had other code readers connected and not been a problem, was most likely faulty ) but i did manage to watch the tps and that seemed to be fine, although i cant remember if it was running ok at the time.

Ive read untill my head hurts, many times had two mechanics look,both said its a case of trial and error, but i cant afford to just keep changing parts.

im going to take a look at the cam and crank sensor resistance, maybe try spray some wd40 around the inlet manifold ( unless theirs a better way to look for vacuum leaks )

also a new problem now, MIL light and p117 showing on self diag, ECT circuit low input, could that make it run rich, and or cause other problems ? temp gauge seems to work fine.

Any suggestions would be appreciated, im more than happy to post results, and any other info if needed

Pat.