Oil pressure gauge shakes at idle but not when in park or driving

I have 2000 Ford excursion,  had new oil
pump put in and sending unit. I pull a small
cargo trailer with it. Do regular oil changes,
probably more than needed but after oil
warms up the oil pressure gauge shakes, no
lights come on and it doesn't drop too low
just the needle shakes at red light . Stops
shaking  when i start driving again or put it
in park. What is going on? I want to keep
this vehicle. It has very high miles over
230xxx