worst comes to worse you "might" have to machine the crank and put new bearings in. If the did a quick and dirty rebuild then they only polished the crank and went back to the original size bearings. This will also cause low oil pressure as the oil is leaking between the crank and bearings, which will not push enough oil up to the lifters and cam's at idle. As Lush said try the pressure switch first. The engine should be under warrenty with only 9K on it. They should cover it for 1-2 years and 20K unless they are a fly by night operation.I have all my machining done out of house but even I cover my rebuilds for a year.