Check Engine Light

[mcoburn6]

Greetings all,
I've been having a problem with the check engine light coming on. Over all I want to narrow the problem down. The check engine light on my Boxster came on last year and I took it to the dealer, the dealer told me that I had 2 old codes, both for o2 sensors. The dealer then told me if it happens again its either a vacuum leak or that I need all new o2 sensors. Well it happened again about 3 months later, so I bought an obd2 reader and read the codes, it flagged the o2 sensors. Then I erased the codes and had no problem until yesterday, it came on, another 3 months later. My question to all, wouldn't o2 sensors flag me right a way and not in 3 month intervals? And if it isn't my o2 sensors, could it be anything else than a vacuum leak?
Thanks,
MC