replace o2 sensors, replace bad injector(s) - search for dremel method if you don't want to pull the plenum. I'd say your oil pressure sending unit is bad or has a bad connection. It connects near the oil filter. Disconnect it, and clean if necessary, reconnect and see if that helps. If not, you prob need a new sending unit. Low oil PSI can/will make your lifters tick. Also, I believe (but not 100%) that clogged PCV's can affect oil PSI too.