rough idle

i have a 94 silverado tbi 350 and i have had problems with the idle so i have changed the fuel filter plugs wires cap rotor iac valve rebuilt the throttle body new egr valve all new vaccum hoses and it still idles bad so i replaced the intake gaskets and set the timing to zero. now it still runs like crap when i checked the timing after i hooked everything back up and the timing is advancing and jumping around between 12 and 17 plus my oil pressure dropped to zero any help would be greatly appreciated i really need to get this thing running right