I had a very similar issue with my 06 1.9mjet. My gearlinkages got so bad it was eventually difficult getting it in 5th and impossible to get into 6th. It started around 90k getting tighter until I had to change them at around 100k.
I replaced both linkages with ones from autodoc, I rang fiat to find the part numbers and was told 55230837 and 55230837 (1 up and down 1 left to right) they wanted £185 plus vat for each cable! I changed both with a friend in a couple of hours for less than the price of one. The ones I removed had very little movement in them and would hardly move, you could see the rust and build up on both ends. Iv got some pictures somewhere, I suspect water was getting in the linkages and over time rusting the internal cable.
I wouldnt be surprised if yours need replacing too, but its always worth checking they are connected to the selectors on the gear box and held tightly in the mounts. If you look just under your battery tray you should be able to see them. Might also be worth watching them move while someone else sits in the car and changes gear and see how much they are actually moving.