Erratic idle on Cavalier 2.0i 8V 1991 model?

Can anyone offer a soltution to erratic idle on Cavalier 2.0i 8V 20NE engine? It is also quite difficult to start when hot. Have cleaned/checked or replaced all fuel lines and cleaned the inlet manifold, butterfly valve and idle air control valve which has also been tested for operation across the battery. The idle is lumpy and erratic at all operating temps by the way. Is the fuel pressure regulator a likely candidate as that hasn't been changed yet? Ta for any help.