Subaru Repair: 99 subaru impreza, valve cover gasket, emissions control

Question
my check engine light came on last month.  It was due for an oil chage about the same time, so I had it changed and coincidently the light went out.  It recently reilluminated and went out shortly there after.  It is back on again today.  Any suggestions?  Also I think that I have a leaky valve cover gasket, I have changed these on cars past, although it seems like more work on this car.  Again, any suggestions?  

Answer
on An OBDII vehicle which is anything after 1996 .A check engine light indicates that the engine control module ECM has seen a problem with an emissions control device or an emissions control device monitoring system.If during a drive cycle the ECM doesn't see a problem the light will no longer be commanded on.Please have it properly Diagnosed by someone who knows Subarus .As while this light is coming on your car may be excessively polluting our environment.The valve covers are a little challenging .

Justin
All Wheel Drive Auto