Re: Is my gearbox cream-crackered?
Sounds pretty similar to the symptoms mine started displaying - started off with difficulty changing gear at high speeds, then onto jumping out of 5th gear and pretty quickly deteriorated until I couldn't select fifth or reverse.
I had the gearbox stripped and it turned out there was a faulty seal which meant that it had run almost dry on oil and that was the root of the problem - i see that you've already had the oil checked though.
Sadly, I was advised that the gearbox was screwed beyond repair and had to have a new box fitted. It's worth getting it looked at first though - I do know that they can sometimes be repaired if caught early. Best of luck mate, hope it turns out OK.