oil gauge needle shake

Went boating yesterday to find that my oil pressure gauge needle was shaking a little at different rpms. Pressure looks good but after a hard run seems that the needle would shake a little like there was a potential problem with the motor. Shows good pressure at wot to be 60 psi and no needle shake but when i slow down to around 40 psi it would wiggle a little and even the low pressure light on the electric gauge would come on and off. At idle and around 20 psi its the worst. Moves the needle from around 1/16 of an inch. At idle the warning light doesnt come on, whats up? I read some were that if this happens at wot, it could be oil foaming and sucking air but pressure is fine wide open. It seams to happen mainly after i have been running hard. Engine is running great with no loss in rpm, but i cant help but wonder if its something wrong in my 540 motor. Gauges are 10 years old could it be the gauge or the sending unit or something else?