I'm more concerned with troubleshooting the p1129 code I just got than the p453, as I think the latter is just a EVAP code that isn't causing me any real problems (although needs to be addressed).

I only drive my S a couple times a week, and when I got in it the other day I noticed the slightest bit of hesitation while accelerating in 2nd gear. Didn't think much of it. Today though, I noticed the hesitation in 1st gear taking off from a stop and it was much more pronounced. It wouldn't accelerate for about a second or two, and then it caught on. When I was almost home, it did it again, but this time as I tried to accelerate in 3rd gear it had no pick up at all for a good 5-10 seconds. I had to pull over to the side, shut the car off and then back on again. It drove fine the mile back to my house, where I've promptly parked it in the garage.

Any troubleshooting thoughts as to what this could be?

 

And to the extent it is helpful, I have a k&n shorty intake, private label header, stock ap2 cat and exhaust. No other engine mods.