The stock gauge is electric and as Doug said, odd things happen. You should have added your new sender with only one (1.0) wrap of pipe tape. More than one wrap is fine for home plumbing but can interfere with the sender's ground to the engine block. Try this: with the engine running at idle, touch a temporary ground wire to the sender body. If the gauge reading changes, remove the sender and excess pipe tape & try again.
Next, turn on the headlights. If the indicated oil pressure drops on your gauge, you likely have a circuit issue, not an oil pressure issue. Start trouble-shooting by first checking the oil pressure gauge ground, then by pulling one fuse at a time until the gauge reading is affected. Start with that circuit and check everything on it starting with the fuse. Somewhere, there's a poor connection or corrosion. Sometimes a separate ground between block and chassis fixes such things. There should be an OEM ground wire between the ZF & frame, but a second one from a cylinder head to frame can sometimes help.