1987 Chevy Celebrity

I bought this car,gave a complete tune-up on it,it cranks up great..runs down the road alright.but when idling,it idles at first like normal,then idles down to almost dieing out,but it doesn't.also when try climbing any hill,it boggs down to nothing,even when the pedal is to the floor,then it dies..wait a bit an it'll crank again.but always with the same result.also after changing my oil,,the check engine light comes on now..A mechanic told me my problem was this sensor thats in between my breatherhose,,part would cost 75$,but couldn't guaranteed it would work..an couldn't return it back if it doesn't,cause it weas electical..Help please!...Jim