A vaccum Leak
Possibly a stuck or sticking EGR valve.
You need to turn off the Overdrive switch.
You might have a dead cylinder or a misfire in one of your cylinders.
What revs should my audi 1.6 show at seventy miles an hour and how many revs should it show when ticking over
If it is running fine at high revs the most likely problem is an injector
Check your radiator is full
Check the Idle Air control valve. This lets air bypass the throttle body plate when the car is at idle or low revs.
Yes it does, mine has exactly the same problem, can be the MAF sensor, fuel pump problems or the idle sensor. just changed the MAF on mine saw some improvements but still revs up and down occasionaly
Try your idle air control valve.
replace the Air Idle Control Valve for starters
add coolant.