low oil pressure

[cedalice]

my 90 ZR1 has only 8900 miles
the car is arrived in europe with 8000 miles in 91 but since 91 only 800 miles.
In 2006 the motor has been rebuilt because there was a lot of water is the tank and when the owner has started the motor there was some damages.
Now the problem is with the oil pressure gauge or may be the oil pressure.
I had a leak from the oil pressure sensor. I have replaced the sensor yesterday.
When the car is cold pressure is around 80 but after 40-50 miles the pressure is low around 0 !!!!!!! but there is no leak under the motor, from the sensor. The oil level is good.
when the oil pressure gauge is around 0 I have no "check gauges" .Before I change the sensor I had this "check gauges" when the car was cold or hot.
All the others gauges are OK oil temp or water temp.
So can you help me ?
and sorry for my english