I just had a head gasket job done last week, and now a few times since I got the car back, it really hesitates when I start. The idle is very rough and drops to the point where it nearly stalls. I try giving it more gas, but it just continues to sputter in the low rev range before finally it kicks the sputtering and reaches normal revs, at which point it runs fine until it rests for a while. Any ideas what it might be? Thanks in advance