Mine does the same mate.

Have you got a fuel cut defender on yours?

Im still running the fuel cut at 1 bar on mine, got it on a bleed valve set to 0.9 bar, and i cant spank it in 5th otherwise it boosts to 1.1 bar and the fuel cut defender kicks in.
Perfectly fine giving it death in 1,2,3 and 4th gear just 5th it has a paddy.

And i also get the ecu light come on when crusing in 4th or 5th, no fault codes present either.