M L'Inspecteur,
Gaston's gearshift began to exhibit the same symptoms and it turned out that the rubber bush at the inside end of the tube had broken and was turned "slightly sideways" thereby adding some friction to the mix.
I was able to first of all remove the offender with needle nose pliers which solved the stickiness at the expense of floppiness and rattliness.
Using the removed bit I cut a replacement from some other, slightly firmer, rubber and by dint of patience, careful insertion and liberal quantities of verbally expressed profanity, was able to insert said replacement successfully. Problem solved, at least for the nonce.
Cheers, Pottsy