You might check the timing belt for tightness. I've seen some engines that will stumble erratically at idle when the timing belt is too loose. This is real easy to do. the way I do it is to first remove the plug out of the timing belt cover that exposes the tensioner pully adjustment bolt. Loosen the bolt enough so the pully will move freely. Then by slightly turning the top (cam) pully bolt counter clockwise (you will need to expose this by removing the top cover retaining bolt) you can remove any slack in the belt and the tensioner pully will reseat itself to tighten the belt. While holding the cam pulley....tighten the tensioner pulley bolt. Don't tighten too tight or the belt will whine. Verify also that the belt is timed properly. Good luck.