hhhmmmmm, seems you have a bit of a confused understanding of exactly what your car is doing, no offence intended!
certain errors that the car picks up on will put the engine into what is called a limp mode. basically on a diesel like you have this means the revs will be limited to about 3,000rpm and the turbo wastegate will effecitvely be wide open, so you don't generate any turbo boost pressure, thus restricting the power.
when people start talking about the MCSF error that means "Motor Control Systems Failure" which is the error message that pops up on the info display when a more serious error is detected. this will also put the yellow engine symbol light on the rev counter. some minor errors that are detected will not put the engine light on or show this error.
what you need to know is what fault code you have generated and therefore stored on your cars computer. a good indie specialist will be able to tell you this if you dont have the laptop and cable to find out for yourself.
from your description its hard to tell whether or not your have an EGR valve fault or if you have a turbo overboost fault. I don't know myself whether an EGR fault would put the car into limp mode or not.
you said that the car felt like it had massive turbo lag, so check when exactly the turbo kicks in. the cars have a VNT turbo, which basically means it can adjust itself to make power across more of the rev range and so limiting lag. you should find you get power from the turbo from about 1,500rpm upwards. if the turbo doesnt begin to function until 3,000rpm then something in your VNT system has failed. don't worry though, its usually only a small rubber vacuum hose and costs about £5 max to fix.
if you are having VNT issues in higher gears its possible that you are setting off overboost faults which will always put the car into limp/safe mode. this typically happens in high gears, 5th and 6th mostly as that is when you are demanding maximum boost for the longest periods so more likely to overboost.
incidentally if you do end up going into limp/safe mode you can turn the cars ignition off and on again and it should run normally again, at least until the fault occurs again and trips the car into safe mode again. edit: just to point out that when i say off and on again I don't mean try any stunts whilst actually driving. you should, if you decide to do so, pull over in a safe place, bring the vehicle to a stop and then turn the ignition off, then restart the engine!
typically after about 6 or 7 engine starts if the fault hasnt shown up during those runs then the engine light will clear but the fault codes should still be stored.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso