how old/many miles ? I've seen a friend's 302 act like this and when we finally got tired of it and pulled/tore down the engine it turned out the main bearings were very badly worn (every one of them was showing copper somewhere). When the mains are badly worn like that the clearance between the bearing & crank goes up dramatically and keeping tight clearances is what allows the engine to build oil pressure. You're seeing 40psi when it first starts because the engine is cold & "tight" but after it runs for a few minutes and warms up the clearances open up and the pressure drops. You see 40psi again up on the highway because your revs have picked up so you're spinning the oil pump faster.
Now it's also possible that the oil pump itself is worn but that usually results in low pressure problems across the whole rpm range.
Unfortunately the cure for oil pressure problems is usually a rebuild. Before going that route you can try changing your oil & replacing it with either 20/50 or a straight weight 50. That's a thicker grade of oil that's more viscous than the 10/30 or 10/40 you're probably running and that extra viscosity will result in higher oil pressure. It's worth a try to see how much of an improvement you get. You could also try replacing the oil pump with a high volume type and see how much of an improvement that nets you. It's not going to fix the problem but realistically, with an oil pressure problem your engine is on borrowed time and anything short of rebuilding or refreshing (a sort of half-ass rebuild where you just replace rings & bearings without really doing any machine-work) is just a way to delay the inevitable.
As far as the temp thing goes, 190* isn't bad. Normal operating them is around 185*-195* overheating temp varies depending on the rating of the radiator cap. Most rad caps will "pop" when the internal pressure reaches 16lbs. They're rated that way because there are too many variables to use temperature, the temp at which the coolant actually boils is going to depend on what the ratio of water-to-antifreeze is. Most caps will blow somewhere around the 240*-250* neighborhood.
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