I bought a second hand 2014 Focus (left hand drive) auto, approx 100,000km on the clock, about 2.5 years ago from a reputable dealer. After a few months it started dropping out of certain  gears - the revs would increase and the gear would shift to either 1st, 3rd or 5th - those were the only gears that worked, no even numbered gears or reverse. Switching off and on again would (after anything up to ten minutes) eventually resolve the issue. The actuator (B I think - apologies, my technical knowledge is minimal) was replaced but the same thing continued to happen, but not on every journey, with no apparent rhyme or reason (I use the car a lot, averaging around 2,000km a month). Last August I took it to an auto transmission specialist who said the fault was with the gearbox computer, and that it had damaged the clutch, which would eventually need replacing. They replaced the computer, and it did indeed resolve the gear shift issues, and earlier this year I replaced the clutch.

However, from December last year a weird and intermittent problem has appeared. Some days (occasionally every day for a few days, often not for a couple of weeks at a time), when I first start the car the engine fires up fine, but it either refuses to select a gear, or will only select first, and will not shift. It can take anything up to a dozen attempts to restart for it to select a gear, after which it runs perfectly for the rest of the day, no matter how many times I stop or start it. It will then work fine for a day or a week, but then do the same thing, and again, it will run fine for the rest of that day, with all the gear changes being smooth and clean.

There don't seem to be any common factors as to when it works or when it doesn't - sometimes after a long trip (100km+) it'll be fine the next day, others it'll be really stubborn to get into gear, other times I'll do a few short runs (less than 20km) and it's the same story. It also doesn't seem to matter if it's been parked in the garage for a couple of days and not used or if it's used every day. It seems to be a little more problematic in the hot weather (summer in southern Spain), but equally it's sat out in the sun in 40 degree heat and worked first time - except on a few occasions when it hasn't, but that could just be coincidence, as it also happens in cooler weather. It happened once while it was with the auto-transmission specialist who said he'd never seen anything like it in twenty years, and had no explanation for it (as far as I'm aware it didn't produce any error codes when he checked)

Does anyone have any ideas as to what the issue could be, or ways of fixing it? Unfortunately I can't afford to replace the car at the moment, so getting rid isn't an option. 

Thanks in advance for any help or advice

Take it to an auto electrician and have ALL the wiring from the gearbox to the TCM and the TCM to the PCM checked for continuity issues.

You have a broken/damaged wire somewhere (probably behind the battery)

Thank you, I'll see if I can get a local recommendation.

Can I ask, why would this cause an intermittent problem? (Not doubting your answer, just curious)

Breaking down wires cause communications issues, sometimes the comms signals get through, sometimes they don't.

Powershi*ts don't have a LIMP/LOS strategy so they just stop working or get confused

Don't bother with garages, go to an auto electrician

And you should improve the battery and alternator ground connexion, it is a know issue, you can look many videos on youtob