next time you have the issue with it not engaging into a gear, where you can pull over and switch the engine off and see whether selecting all the gears is still as difficult as with the motor running.
if there is no change and its just as difficult this will rule-out the "clutch release" system as being at fault. for example if it is easy to select the gears once the engines off then it sound like the clutch cable has stretched or the clutch arm is partially seized where it enters the gearbox. (which could of caused the cable to then stretch for example)
if there is no change and its just as difficult this will rule-out the "clutch release" system as being at fault. for example if it is easy to select the gears once the engines off then it sound like the clutch cable has stretched or the clutch arm is partially seized where it enters the gearbox. (which could of caused the cable to then stretch for example)