First Technical Question - Islero Oil Press Gauge

Gentlemen, since your 400s are very similar to my Islero, perhaps you might be able to point me in the right direction with this.I have only put a few miles on my Islero since I bought it (haven"t had time to get it inspected yet), but I was driving it yesterday and noted the following.With the oil fully warm, at idle it registers about 6- 8 kg/cm2.As I get above about 4500 rpm, the needle intermittently fluctuates wildly, the needle bouncing all over.I though it was probably just the gauge.But then a particularly troublesome thing happened after I accelerated strongly away from a light (showing off).The gauge dropped to zero with throttle, then returned to "normal" when I backed off.Even part throttle would cause it to swing back to near zero.It was like the silly fuel economy gauge in my old 944. I"ve never had a car "lose" pressure as the revs go up.Any ideas where to start diagnosing the problem?Could the sender be bad (it is 33yrs old if original)?Could the pump be bad?Wiring?Gauge? Awaiting your wisdom, Udo Schuelke Islero 6198