Gearbox Hard to use in reverse or 5th when hot

Hi all,

As above on a Fiat coupe 20v turbo. All gears are fine to select when the car is cold, once warmed up or sitting in traffic for 10 - 15 mins reverse and 5th become very difficult to get out of, sometimes I can reverse into a space and not be able to get out of reverse until the car cools down.

I have checked gearbox oil which is fine, no crunches or dropping out of gear just the problem above. 1st through to 4th are fine.

Any ideas what could be the issue?

Thanks

Austin