'98 1.0L Corsa: intermittent Check Engine - Air Flow Meter

Hello,

I am a newbie to motoring, but I have a problem that I apparently need to research with my own hands (garages either want to pump money by replacing everything in sequence, or tell me not to bother).

I have a 1.0L 1998 Opel Corsa; right hand drive as common here in Ireland. The car runs fine; has a few known problems which I'll list later - first, the one I'm struggling with.

The Check Engine sometimes lights up for a time. And I know how to reliably "make" it light up:

- get up into high gear. 5th will do the trick all the time, 4th sometimes

- speed up so that the speed is not low for the gear. About 60 kmh for 4th, about 90 kmh for 5th

- release the accelerator pedal and let the car slow down gradually without changinggear (or sometimes, *slightly* press the brake)

This brings the Check Engine light on. Then if I press the accelerator (with otrr without changing the gear down), the light goes off again.

I took the car to a garage, where they took a computer reading which said "Air Flow Meter". (I think it was "low voltage"). They told me to replace the air flow meter; their price was through the roof but I found a cheaper one (insert only) in a shop and got it fitted. No change!

I need to get rid of this as I want to use the car for a driving test, and one can be failed on the test if "a system warning light comes on at any time".

Here are the known problems, in case any of them may point to the cause:

- the accelerator is somewhat stiff compared to newer cars (but still manageable)

- a compression check (in a garage) showed low compression (100 psi) in one of the cylinders; but the car still has enough power to run for now

- the battery is weak; if the car is left alone for several days, it needs a jumpstart to get moving again

What should I check for, try, etc? I have a more experienced friend who can help me with the tricky manual work, but I need directions.

WIll be most thankful for any advice on fixing this.