Need some help w/ two new codes for O2 sensor & EVAP

[Briandroy]

I have two error codes and am wondering if they are related somehow. The codes are:
P0442 - EVAP emission control system leak (small)
P0133 - O2 sensor CKT slow to response (bank 1 sensor 1)

The research I have been able to do has indicated that the p0442 code alone could indicate something as simple as the gasket on the gas cap, which does have some cracks in it. I have also found out that O2 sensors are VERY expensive. I do not want to just start replacing parts all willy-nilly given the history I have had with my car.

The recent history, since January of this year, which may give a clue to the problems. Spark plug blew out of the cylinder head, repaired, oil leaks in PCV and Turbo drain seals developed and were repaired. Timing set incorrectly, repaired.
The symptoms now are: the car seems to be 'missing' while idling and started rough, read the OBD codes and came up with timing adjustment needed. Took it to the dealer had that fixed, the car no longer starts hard but continues to 'miss' periodically while idling and also seems to sporadically lose power while driving. This power loss is rapid and isolated, not continuous.

Any advice would be greatly appreciated.