256 euros for an injector harness? They are having a laff, they should cost about £40 and you can fit it yourself with a couple of sockets and no more than an hour's labour, the harness doesn't need anything fancy to fit only if you change injectors or ECU. But the injector harness misfire is usually absent when cold, then gets worse when hot and stays worse, all the ones I can recall seeing are quite predictable like that, once it starts to fart and jump around it stays farting and jumping, doesn't come and go while driving.
It could be a dud ECU, they do go pop, though mostly only the very early ones 99/00 sort of vintage.
It could also be a problem with the crank sensor, as if the ECU loses the engine speed/position signal it will have a panic attack
Really you need to know what fault codes are logged otherwise you are a bit in the dark with tracing it. I don't know what a dealer would charge to give you a fault code check but it only takes 5 minutes to do so should not be too pricey? Doesn't need to be a main dealer, could be done by an independent with Autologic or similar kit.