Vacum leak?

[zmanpro63]

So Ive been doing some searching and reading of posts and Im wondering if my problem could be a vacum leak? I have a 99ES with AWR header (1st cat removed) and an AEM SRI. when the car is cold everything is fine but when the car is warm it starts to do something strange at idle. When warm she normally idles around 900 RPM or whatever the first line is below 1000 RPM, but the longer you sit at a light or are just idling the rpms will suddenly dip like its gonna stall but just as quickly it catches up and occasionally it will just idle rough at around 500 or 600 but if you give it a little gas it resumes its normal idle. Now my CEL is on but I have assumed it was just from the header and my MIL eliminator not doing something right, but I havent checked the code since I last replaced the second O2 sensor (last Friday). But I remember my mechanic telling me a month or so ago that it was actually throwing two codes, one for the O2 sensor and the other being that the car was running lean? I think maybe rich but Im pretty sure he said lean. Now I have been rear ended twice in this car and I have never had any problems with the SRI, but is it possible that I have a vacum leak or maybe an egr problem? Maybe something got loose from one of the accidents? They were both fairly serious damage wise the first was $5500 and the second $3900, thanks for any and all help Nik