90 km/h is highway speed, isn't it?

It certainly sounds intermittent. Then again, i almost wouldn't rule out a slipping clutch, like it's not engaging all the way or something, but you'd know pretty well if it was that.

what i meant, was while the motor is lumbering, hold the clutch to take the load off the motor and see if it lumbers still without the full load

maybe somebody else can chime in?

 

5th gear is really long and even with a V8 needs to be dropped down to 4th on some hills...