won't stay running, without help

I'm a new owner of a 99 GTP and I love it. The first couple days of ownership I had no problems. Now every time I start it after it has been sitting for 4-5 hrs or more it wants to stall out. It starts up good and begins to sit at idle and once it hits 1000-850 rpm it dies, if I keep the engine revved up and get it warm (1-2 min) it runs great. One other thing I noticed is the oil pressure gauge is all the way down during this time once I drive it for a minute or so it builds pressure. Any similar problems ?