C2 VTS - help me diagnose

[PlsNotAgain]

Hello everyone,

first post on here, hopefully I'm doing this right.

So to summarize my C2 VTS (late 2005 model, 230k km) ownership, I've been having some troubles with;
1) rough starts, sometimes it cranks infinitely but doesn't start and then on second crank it fires right up
2) rough idle, sometimes perfect idle interrupted by tiny "jerks" you can feel through the seat, sometimes its hunting for idle (not a lot but enough to hear it)
3) it started doing this about 4 months ago, after sitting for a day or two, I would drive about a kilometer and i would get an "Engine management system faulty" + service light which would put me in a soft limp mode (it went over 3k rpm but very slowly) which would go away after turning the car on and off on a red light

So with all that in mind I started hunting those problems down, first changing sparkplugs, ignition coil, crankshaft sensor, engine temp sensor, MAP sensor...
I was planning on getting a new fuel pump too but then I re-scanned the DTC's and got P1153 - Program Motorised Throttle Limits Lower Limit, and on a different occasion when I (again) got the EngMan SysFaulty, I scanned P0121 - Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Preformance.

After that I went digging, reading live data for the throttle body I got:
- "Absolute throttle position: 22%" when I wasn't touching the pedal, so "idle" - Shouldn't it be closer to 5ish %?
- "Absolute throttle position: 77%" when gas pedal to the floor - Shouldn't it be closer to 95-100%?

- Live fuel trims were all over the place, -18 +12 constantly, which corresponds to the car "hunting" for idle.

After which I took it off, cleaned it, cleaned the connectors which were spotless, but got the same results again.

Finally, if anyone could help me, and tell me if those errors prove that the throttle body is at fault so I don't go buying one if there is no need.
(I also thought that it might be the gas pedal but that should be throwing p0122 or at least "B", correct me if I'm wrong)

Thanks for reading.

[PlsNotAgain]

Ah, I also forgot to ask.

What is the recommended way to reset the ECU and Recalibrate the throttle body after changing it (if I were to change it), without a Lexia/Diagbox or other high end diagnostic tools?
I've seen a lot of methods be thrown around so I just want to make sure I get the right info.

[Colnago1]

I will suspect at pedal failure,
reset BSI quide
Put the driver's window down, lift the bonnet and ensure all equipment is switched off.
Ensure all doors are closed and remove key from the ignition.
Wait for 3 minutes, disconnect the vehicle battery and wait at least a minute.
Reconnect the vehicle battery, wait a further minute (do not open doors).
Switch on the side lights through the driver's window.
Switch on the ignition and check system's functionality.
Hold lock button on key down for 10 seconds.
Remove key open & close door test central locking system.
Start the engine and complete the system's check.
Failure to follow this procedure could result in incorrect operation of many BSI related items.

[ozvtr]

Unfortunately these faults tend to point to the engine ECU.
The electric throttle bodies and the accelerator pedals tend to be bullet proof! There is a very small possibility of a throttle body failure but an almost zero chance of an accelerator peddle failure!
Apart from an internal failure of the engine ECU, you could inspect the electrical connectors at the engine ECU for moisture and corrosion of the electrical contacts.
These above faults will set the engine light and put the engine in "limp mode".
However they wont cause rough idle, hard starts and misfires. The misfires should be logged in the engine ECU, but they may not bring on the engine light.
Other things you could look at are fuel rail pressure and the injectors.
If the fuel economy and general performance are good then I wouldn't suspect the O2 sensor.
The VTS has an aluminium inlet plenum, which tends to cause the nuts holding the plenum to the cylinder head to work loose and cause vacuum leaks. Check the nuts for tightness.
Vacuum leaks tend to effect the idle, "bottom end" and starting but do not really effect "top end".
Do the fuel trims settle down at cruising speeds? Or still remain erratic?