97 suburban oil pressure

1997 GMC
suburban
5.7 l v-8
178000


My oil pressure gauge is acting up. When I first start it it's fine, but as I accelerate the pressure rises(up to 40psi), but if I slow down or stop it plummets to 0psi. Do I just have a gauge going out or do I need to look at an oil pump. Anyway to troubleshoot this, I'm not much of a mechanic outside of light maintenance.
Connect a mechanical gauge and take the reading that way.
This will determine if there is an engine oiling problem or just a gauge and/or sender problem.