2002 Jeep Cherokee low oil pressure

2002 Jeep Cherokee Mileage: 46,000. My Jeep recently began dropping oil pressure to below 10 psi at idle makig the check gauges light come on. We took it to the dealer, they checked the sending unit which was fine and ended up replacing the oil pump. For a couple of weeks the pressure was between 13-15 psi at idle (we were told by the mechanic that that was ok) and now the pressure has begun to drop to 10 psi or less at idle causing the check gauges light to come on again. The vehicle is running fine otherwise. Any ideas?