The thing is your ute might have a tune in it unbeknown to you. It just wont take 91 under load.

^ this in addition to one or many of the following;

Dirty Throttle body,

dirty injectors

dirty fuel filer

fuel pump on its way out

boost solenoid could be screwy

turbo bolts loose - can cause all of this due to lack of boost and overcompensating with fuel

lack of oil pressure

basically all the overlooked things in your regular servicing schedule by poor mechanics can result to the computer over-correcting everything as it is all computer controlled.

best job you can do is go over all of these yourself as it is all cheap and easy stuff to do in the comfort of your own shed, and then when you go to the mechanic you can rule all of these simple things out and not get charges out the arse for them.

Could even be valve springs but its all guessing. Start with easy stuff and make sure no leaks missing bolts. And as stated could have a tune for all you know.

As for 91 my understanding was only if you cant get anything else and dont boost it as you should run a min of 95 for giving the thing abuse.