My father-in-law gave me his old 89 S15 Jimmy to fix up and sell. Only thing I can find wrong with the motor is the oil pressure. You start it up and you have nice good pressure(about 65), but after you drive for about 45 min, the pressure has slowly dropped to "0". Shut the rig off for about a min, start it back up and you have full pressure again and can drive for about another 45 min when you've lost pressure again. Any ideas?