Friction-shift (rear) always changing up the gears I have an old french touring bike (from the 70s or 80s) with downtube friction shifters.

I recently had it serviced, and ever since there's been a constant pressure on the rear gears which means they have a propensity to shift into the highest gear. If I shift into a lower gear and let go of the shifter, the chain works it way back up the gears. I can generally maintain being in the second highest gear (which is the one I use most often) but it still requires periodic movement of the shifter as it moves its way forward.

It's possible that before the service the mechanism just wasn't lubricated enough for this to happen.

Does anyone know what's causing it and how I can fix it? I'm afraid I'm pretty much a beginner.
Rode downtube friction shifters bikes for a while. 
I can see two possible causes : either the shifter was too lubricated, or the screw isn't tightened enough. Try to tighten it more, but pay attention not to tear the screw. 
Usually the lever is pretty rigid when moving. If it's easy to move it to the point it moves alone, it's probably because it wasn't tightened properly.
If this does not work, you'll have to unmount it and delubricate the parts a bit.