Hello, I see you are still waiting, I will try to assist you although my knowledge of Vauxhall is somewhat limited and you may need to bear with me while I find data as required.
I've had a look at 1.7 cdti turbo's and it seems there is either a wastegate control or VVT.
It seems 2009 may of been a change over year also. If you have an engine code that may help.
The underboost conditions you describe, lead me to think there is something wrong with either the wastegate or variable vane system, depending on which system you have.
The overboost condition you describe makes me think that there is a problem with the VVT and therefore it's VVT.
My thinking here is that with the lack of power initially and we know it isn't egr, that for a waste gate operated turbo, this would cause a slow spool up that would generate some boost in the end but not near normal pressures. For a variable vane turbo, if the vanes are stuck in the wrong position then this could cause a extended spool up time leading to overboost in the end.
Let me know which you've got and I can be more specific.
If it has VVT then it sounds like either the vacuum solenoid, vacuum pipes, vacuum pump (how's the brake servo assistance?) or the vane control arm have issues.
The idea of the variable vane is that it transitions from a quick spooling low pressure turbo through to a essentially late spooling high pressure turbo, by adjusting the pitch of the vanes, this helps it spool up quickly and then go through to higher boost levels rather than, wait - wait and here we go like the older style fixed vane and wastegate turbos.
If the vane is stuck in a high boost position, it would have long lag then overboost.
I would see if you can detatch the control arm and move it if it is a VVT then check the vacuum system and solenoid.
I would think the other codes are related to the P0234 issue, torque reduction for safety to the transmission.
A picture of it would be handy for me to identify it if possible.