Citroen C4 HDi automatic 6-speed gearbox

I bought this recently and gave it a nice going-over — it had been neglected, basically just a bare engine and chassis, the rest was left to me. I started stripping it down completely. Front and rear control arms, bushings, brakes, after the steering rack — the seat had also collapsed so I redid that too with the original foam, such a shame because the car is actually really comfortable. Anyway, next in line for maintenance was the clutch pressure plate and disc. It's probably from an old engine swap and that part had just reached the end of its life. While replacing the pressure plate and flywheel as a set, I figured I'd give the transmission a full service too — I changed the gearbox oil and the mechatronic (robot) oil, and did it again a second time not long after. But the problem wasn't solved. It's doing this dumb slipping thing at steady cruising speed, like the old pressure plate again. It didn't do this in 3rd, 4th, 5th gear before — now when the car accelerates gently and you rev up, the pressure plate half-slips to give a kind of acceleration, and it only does this in normal mode, not in S mode or manual. And every once in a while, after pulling away from a red light it shifts from 1st to 2nd, then just as it's about to go into 3rd, somewhere between 35/40 km/h it drops back into 1st — the engine screams close to the rev limiter, I shift it up right away but it's really not healthy. When I take it to a mechanic I can't really explain this, and it doesn't act up in front of them either. Has anyone experienced the same thing and knows what it could be?