oil pressure guage

I have a 1997 C1500 5.7L with an oil cooler. Recently I have noticed that the oil pressure guage pops around a bunch. When warmed up it sits at 20 psi but will bounce between 18-25 when on the highway going 75 it sits at about 42psi and will shoot up to 50 and then come back down to 42ish... Any thoughts? I notice nothing else happening (RPMs constant, no knocking in the engine, oil is full and nice color.)
My truck also has 125,000miles and I bought it at 85,000...I have never had it "tuned up" and I dont know when the last time this was done. Does it need it and how much will it cost. What do they do in a tune up? Thanks Guys