Golf III TDI doesn't accelerate smoothly

Hello everyone,

My Golf III TDI, manufactured in 09/94, with 180,000 km, isn't running as smoothly as it should, and I've noticed this for quite a while (over a year), but it only happens in 5th gear, starting around 140 km/h.

You can barely notice it, so there's no loss of speed or anything like that, but you can definitely feel it.
To me, as someone who knows little about cars, it sounds like a clutch issue. However, I wouldn't say that the clutch is completely slipping.

I took my car to a workshop, and they test drove it, but they said they 'didn't find anything' wrong. Well... maybe it was the wrong workshop, or the 'showroom effect,' but I'm definitely not imagining things.
What do you think, clutch? Solution? If the clutch needs to be replaced, what is the cost?

Thank you and best regards.
Jorainbo.

Deleted as requested.