Yeah brembo449 is right, sounds like clutch slip to me. On my old 1.9 GT it did exactly what you've described, ran it for about 2/3 weeks, slowly crept into 5th, then 4th. Finally took her to my local independent and the clutch was down to the bolts that hold the plates in, basically, way way past its sell by date
They didn't know how it was working at all! so it seems that they last right to the very end!
How many miles on the odo, and is the clutch bite point high on the pedal? Mine was high and went at around 83k i think, GT's lighter though so i would suspect it could be sooner maybe 70k on the 159 with the extra weight.
I'd wait until it starts slipping without cruise on in 6th maybe 5th then you can be sure it is the clutch and not some weird electrical fault.
You could also try this.. .(not advisable tbh but it worked for me), with car stationary, pop it in 4th come off the clutch gently with medium revs, if its on it way out it will most definitely slip and you should get a good whiff of burning clutch, if not and its got some life left it will set off/stall, if you dip the clutch at the right point it wont cause any damage - others on here might advise not to do this lol
FYI - new clutch kit, gear oil, dual mass flywheel, bearings, labor etc etc is probably going to be close to £1k reckon, if your keeping the car, make sure you get the DMF replacing too while the clutch is out.