1992 Dodge Stealth Low Oil Pressure

I have a 1992 Dodge Stealth with a 3.0L DOHC engine and 239,000 miles. A couple of weeks ago I changed oil and switched from 10-30 oil to 5-30 oil since it has been very cold recently. It ran fine for two weeks and then the oil pressure started acting wierd. I started it and the oil pressure didn't go up and after 10-15 seconds I shut it off. I started it again and it did the same thing. I started it a third time and the oil pressure guage moved a hair and when I revved it, it jumped to normal oil pressure. It has done this every time I drive the car, except sometime it takes much longer to get oil pressure and revving it when I get a little pressure no longer does any good. THe lifters also used to make noise for a few seconds before I got pressure, and they no longer make noise when I start it. What could be causing this? Could it be a faulty guage? (Tapping the guage does nothing) I really don't know what would cause this. Any ideas would be greatly appreciate. Thanks!