Any thoughts on what could be a likely cause for a slightly bumpy first/lowest gear?

1950's Monark - Maillard Atom 66 (5-speed) freewheel - Huret Allvit derailleur

The chain is less than two years old (tension looks good to me) and the cogs look to be in nice shape visually - the first gear is the most commonly used one though.

The chainline is dead straight and the chain isn't hitting the spokes - switching gears is crisp and effortless and there is never any skipping. Changing to the second gear and then onward, everything is operating smoothly - the slightly bumpy/uneven feel only affects the first gear and only while pedaling; when just turning it by hand it feels smooth.

Two years ago I completely serviced the Huret Allvit, cleaned/re-packed the bearings etc. I'm not sure whether the distance from the upper derailleur pulley to the largest freewheel cog is perfect, but it looks ok to me. If I'm not mistaken there's no way to change that distance with this particular derailleur.

Any ideas?