Oil Pressue Guage

My 1970 280SE's oil pressure gauge has recently started fluctuating. Normally after an oil change, the gauge would be to the top (45) upon start of the car and even when coming to a stop.

Recently, the gauge will drop to 15 when coming to a stop. The engine feels slightly "sluggish" when accelerating until the gauge is again to the top (45), which takes about 3 seconds. Difference here is the drop to 15, sluggish engine, and delay in pressure. Previously, even if there was a drop in pressure, the engine wasn't sluggish and the pressure was restored with a second of accelerating.

I've just had an oil change (is 5 quarts enough for this 6 cyl engine?) with new fuel, oil, and air filter. She's running smoother (air filter was ugly) but this oil pressure thing seems to be either fuel/air mixture problem...or compression. Any ideas?