Self governing montecarlo 1.6tdi?

Hello, after trolling thru 20 pages can't find my problem. We have a Fabia Monte Carlo tdi 105ps, 2013, 12k miles. It's just been back to agent for warranty work but one problem we have they say (not exactly doesn't exist) was alright. Couldn't find a problem. The problem being that when travelling at say 40-50mph in 5th, I then ease off and then at around 32mph, still in 5th (roundabout or junction) it starts accelerating again, without any help from me, up to 36mph, slows down to 32 then back to 36.  It also keeps this speed, unaided, when going up slight hill, a flyover. It's as if a govener kicked in.  Agents asked about short jouneys. The problem started about a month ago when we had occasion to make a round trip of 80 miles every other day. speeds up to 70mph. Now this is cut back to 1 or 2 times a week.  Our other journeys are usually short, anything from 5-10 miles. Anything constructive plse that I can take back to the agents. Many thanks.