temp & oil presure gauges sporatic

[Mikeopsycho]

I've got 1979 twin merc 470's. The starboard temp and oil pres gauges only work sometimes. I had the engine running yesterday and at 1000 rpm I got no reading on both gauges, at both the upper and lower helms. When I dropped her down to 700 rpm the gauges started working, but when I rev her up a bit they quit again. Weird. Would this be a loose connection somewhere? How are these gauges interconnected? I'm not sure where to start looking. Thanks.