Speratic oil guage problem

I have a 2000 dodge 1500 ram 2wd w/5.9l engine with 105,000 miles. While Driving down the highway after the engine has warmed up the oil pressure gauge speraticlly goes from normal(about 3/4 at steady 2000 rpm) to just less than half(still at a steady 2000 rpm) and then back to just about normal. There is no change when the Truck is sitting at idle. When the gauge changes I also hear a pinging noise like the valves are slapping. There is no noticable change in performance or rpm's. Could someone please help me try and figure this out. I and a handfull of tech friends are stumped. Thanks, Jerry