Erratic oil pressure - 94 Trooper

My oil gauge shows an erratic pressure, going from very low to very high. Engine performs normally. I change my oil every 3k miles. What can be causing this?

the oil pickup on the oil pump could be full of gunk.

OR

the oil pressure sending unit is bad.

Can an additive such as "Restore" be causing this? I've used "Restore" during the last 3 or 4 oil changes. Thanks.

Is this a serious/expensive problem? What can I expect? Thanks.