sp232007

Hi I'm at a point where I may need to take this to a Transmission Shop.
When the car warms up, selecting gears becomes hard .
When cold moving the gear stick left or right, it will return to centre and I can select every gear.
When the car warms up it doesn't.
Ive done the following :
- Checked, cleaned & re-greased the ends of the shifting cables.
- Checked the neutral position/adjustment of the cable.
- Cleaned and re- greased the ball joint on the selector positioned above the gear box.
- Changed the gear box oil to Castrol VMX-M ( thinking of adding a tube of Nylon G70 ).
- Replaced the brake/clutch fluid ( note tiny air bubbles still present when flushing the clutch fluid ).

I'm thinking perhaps my problem could be one of the following :
- gear cables sticking and should be replaced .
- Clutch master cylinder or slave cylinder letting air in the lines ( no signs of a leak and fluid level not dropping ).
- Clutch needs replacing ( car has done 250000kms).

Has anyone experienced anything similar and what was the fix.
I don't think its the gearbox as I can select the gears when the car is cold ( first 0- 15minutes).