(T) Gear selection problem

Our Multi 1.6 ELX intermittently has difficultly going into 4th gear (no crunch just needs a bit of force) and then it wont come out of 4th without a struggle.

After much testing (we have had it over 12 months) I have now discovered that this happens whenever the the car is accelerated beyond 5000 rpm.

It will happen even if the car goes over 5000rpm in first. Numerous gear changes thought 2nd and 3rd can be made ok until I try to change to 4th gear then it will happen.

Obvously this is can be exceedingly dangerous when it comes to over taking.

Unfortunately, while the car was in warranty we couln't get the garage to reproduce it. Now it is out of warranty.

Anyone else have this problem?