2004 Subaru Outback Misfiring at low rpms

Earlier this week I had my valve gasket covers, spark plugs, and spark plug wires replaced. But just yesterday my check engine light came back on. The CEL only flashes briefly when I'm decelerating or idle but when the rpm's get to about 2,000 the CEL becomes solid again. I'm more worried about the misfiring at at low RPMs than the actual check engine light. I'm sure my CC is busted from previous misfiring. Any way to spot what may be causing the the misfiring now that it's just occurring at low rpms?

Thanks