There's a chance that your 'bottom end'-the crank and bearings is so worn it can't carry oil pressure when the oil is warm.
But here are a couple things to check: When was the last time you changed the oil? Did you use 30 wt or did you use a multi-weight oil like 10w30? they tend to loose their multi-viscosity properties when they get old. Another thought: Does your jet3 have a hydraulic pump in the left engine side cover? Is the hydraulic fluid getting gone? If so, your seal is leaking in the pump and they hydraulic fluid is getting into the oil and diluting it. The other thing to check is the relief valve in the oil pump. It is on the right hand side of the oil pump-The oil pump is at the front of the engine oil pan, on the bottom. There will be a big nut with a setscrew in the center, locked by a nut. You can take the big nut off and remove the spring cap, spring, and plunger. If the plunger won't come out, it could be sticking in the bore and not regulating the oil pressure properly. put a bucket under it-if there is oil in the crankcase it will leak until its out or you get it back together.
you can also try turning the screw in and see if you can get the oil pressure to come up a bit, but I would reccommend taking it apart and making sure the spring and plunger are o.k. first.
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