UK Car Repair Questions? Ask a Mechanic for Answers ASAP
How often do you do 6000rpm in 4th gear?
How old is the new transmission now, and does it have a warranty?
I wouldn't say it is common Josh.
At 100 miles old, I would be ringing the trans shop and explaining your situation.
Sounds like you have a baulking 5th synchro, if 5th is the only problem.
If 3rd is also still doing this, then I'd say there is a mainshaft endfloat issue.
Hope this helps.
How are you going with this?
I hope my advice has been of assistance to you.
I would agree with that.
I would be a little more gentle on the box for a few hundred miles.