Vauxhall Corsa 2000 1.2 runs juddery

Hi,

I have a 2000 Vauxhall Corsa with a 1.2 Ecotec engine. Had some engine management light issues - garage said coil pack which we replaced but light still on. Diagnostic codes saying lambda sensor, replaced and light still on. Some research on google threw up the mass air flow sensor which we replaced and 'bingo' no light. However now when you drive it, it runs ok then starts to run juddery as you accelerate. So any ideas what it could be?

Plugs and fuel filter have been changed as well.