Vauxhall Corsa coughs splutters no power

i bought car as a fixerupa. had work done and it ran lovely. then started to run really rough sounds like exhuast blowing from front but all new so it not that. it coughs splutters engine light on. holds back. had it put on diagnostic and egr valve came up, replaced it it was fine for 4 days then same problem, diagnostic says egr valve again. car had been off the road for a few months when i bought it. mechanic tells me that egr valve must be faulty cos it sticking open. every now and again it clears itself and runs propperly again but couple of miles and same problem. have bin told by ex aa man that cos it three cylinder some diagnostic computers can't read it correctly, but can't afford to take it to vauxhall. any ideas??? i'm at my wits end with it every time i think its sorted it starts acting up again