Oil Pressure on 1992 Yami T9.9

[Endtuition]

Have a 1992 Yami T9.9 four stroke. It has a low oil pressure control to help avoid self-destruction. The weird thing is the oil pressure starts out OK and then seems to drop after about 30 seconds. Cut it off, Immediately restart it and you get another 30 seconds. What would make the oil pressure drop if it's OK when you start it? I replaced the pressure-sending unit (switch) and it made no difference. I double-checked the oil level to be sure I wasn't foaming up the oil. I can't check compression because the plugs are very small, and I don't have an adapter that fits and is long enough. Must be a special tool. Anyone with some insight into this?<br />Thanks in advance.