1999 Vauxhall Corsa engine management light and loss of engine power..

Hello,
I have a vauxhall corsa t reg(1999) 1.0 gls..

And I can't remember exactly when it started but a couple weeks ago, the car wouldn't start.. it would start then like jerk abit then turn off.. I got my local mechanic to have a look and he said it was the air flow sensor and the engine management light was also on..

He disconnected the wire that connected into the air flow sensor and to my amazement the car started and I could even drive it.. (even though it would like slow down suddenly and I would have to quickly go to a lower gear.. the car wouldnt stall.. it just suddenly now again struggles to pick up speed.. only in gears 1-3 in 4 and 5 it all okay..

After hearing that the air flow sensor had gone.. I went to a few shops and asked how much for just the part, no labour I got ridiculous prices like £218.50.. I searched around online and found 1 for £35.80 including insurance and special delivery off of virtualvillage.co.uk

The part came recently and got it fitted and it seemed like the problem had gone.. and the engine management light disappeared and it seemed all ok..
Until like 30-45minutes later the car engine light came back on and started to do the same thing where it would struggle to pick up speed.. but more worse and more regularly..

I disconnected the wire again and the car works much better, even though it still does the slowing down thing..

Does anyone have any ideas on what it could be?
Please get in touch

Thanks
Kizert