Firebird starts but acts like it's going to die

2000 Firebird 3800 V6 around 61,400 miles
A couple months ago whenever you started it, it was cutting out like it was going to die. The spark plugs and spark plug wires were changed and it seemed to do better. Then I went out of town in it and it died on me. A new battery got me home and everything seemed ok. Now it's doing the same thing, when I start it up or have to come to a stop, it feels like it's going to die. Today I noticed the oil pressure gage jumping -- it didn't go high or low, but stayed in the middle and kinda flickered. I had it checked out before my trip and the mechanic said it seemed to be in really good shape for a car its age.