5th gear loss of power

[philco]

I have been doing a few k's on the M1 to work this week, it pulls well up to 95-100k in 4th but when go into 5th gear start to loose power esp if on a slight incline, don't know what is going on, last oil change was 9000k ago, due to do filter and oil again soon, any ideas?

[TheLowRanger]

Which vehicle/motor. My V8 D1 has been doing that for a while now, but living in the Kimberley no-one wants to look if it's not Toyota. I am under the impression that I may need a top end rebuild, but haven't had this confirmed in any way.