VW Golf MK4 1999 1.6SR 8v Problems!

Hi, i'm new to this kind of thing, so here it goes

I have a 1999 golf MK4 1.6 8v
75,000 miles

It recently had its MOT and flew straight through, however just a couple of days later and i now have all sorts of problems

After reading the fault codes through a VAG-COM i have had the following replaced -
New Mass Air Flow Meter (MAF)
New Lambda Sensor
Throttle Body has been replaced
New spark plugs
New Thermostat

This was because of very poor fuel economy (getting 60 miles out of £20) and the car was idling high and erratically, it would basically track up and down and usually settle anywhere between 1000-2000rpm, and will also loose power under accelaration, feels like someone hitting the brakes, then it will pick up again, no power going up hill and top gears have very little response.

after having all of these things replaced i am still getting very poor fuel economy and the idle still wont settle, the throttle body has been switched to basic setting however this has made no difference.

The following fault codes keep coming back through the VAG-COM

00533 Idle speed regulation
19-00 lower limit

00537 Lambda regulation
19-10 lower limit

Every time they will come back with either 'lower limit' or 'lower limit intermittent' i have searched endlessly through other forums to try and find a solution but i dont know what else to look for, i have also just had a full vacuum leak test done and there are no leaks, the car will start and idle normally bust as soon as you touch the accelarator its basically does what it wants, that combined with the poor fuel economy, my mechanic is now telling me that it could either be the ECU or the Wiring Loom but to test and replace these elements would take days so he is unwilling to spend the time on it!

Any help would be much appreciated as i dont know what else to check for and dont want to get rid of the car but i am only a student and i cant afford to keep it running at this rate!