Sounds like two separate issues but associated with each other
1. Bleed the hydraulics , it wont do any harm and should be done every two years anyway!
2. Check the gearbox mount is the gearbox lurching forward more than the engine block when jumping the biting point on the clutch?( the mount under the battery tray)... If it's worn it will jump out of gear easily as it pulls on the selector cables.
3. If the mount is ok, get the selector block on the top of the gearbox inspected by an Indie for proper alignment. You will see the cables running to it.
If it was a worn clutch, fluid or internal issue it would jump out in all gears not just third and fifth. so number 3 seems the likely culprit
75w-80 gearbox oil. 12mm allen key IIRC... Filler is next to reverse switch facing forward, make sure you can undo this before you drain the oil, the drain plug is 10mm IIRC allen key and is the lowest plug on the box under the drive shaft, passenger side. Fill until it starts to dribble out the filler hole. Again, should be changed every two year anyway!