The CEL that won't die
 

CEL 1276 (fuel metering control) and 0153 (bank 2 sensor 1 ckt slow response).

First came up before I replaced the upstream O2 a long time ago. Codes came back.

Replaced the MAF, codes came back. My mechanic checks with Autologic thing, zilch.

Replaced the headers after finding hairline cracks at joints. Went ahead and replaced all four 02's at 100K miles. (cough EXPENSIVE)... Codes came back.

I clear the codes with the scanner, hit a dip in the road on the highway, CEL comes on.
Frayed wiring? This has been going forever...

PL - Stock or aftermarket headers?

Aftermarket. But I was getting this same CEL well before I ditched the stock units.

Throwing parts at a car tp fix never works ;)

The code you have does not indicate a bad MAF sensor !
And the codes come in an ordee for a reason, usually the main problem is the first cause, and the other are caused by this problem.

Go to a good garage and have them diagnose it.

Edit:
I've just checked, and the problem for P1276 is "signal delay time for oxygen sensor ageing, bank2, above or below limits".
So
Check all the o2 sensor wiring and connections. Then change the o2 sensor left to right and right to left. Then erase the codes.
When the codes come back, if the code is still P1276, then it comes from somewhere else, can be an exhaust leak at the head, or a leak anywhere but in the intake that leads to more air in the exhaust.
If it comes back as P1275, then the O2 sensor might be the problem, wire, connection, oxydation... Check it.

^Except for the first 02 sensor (my idea), we did not change those parts because of the CEL itself, they were replaced for other reasons (different CEL codes I can't remember right now). I guess the way I wrote my initial post made it sound like I was replacing parts just to clear the codes, but that wasn't the case.

The O2 connections were checked before the decision was made to replace all four sensors, so the code itself doesn't make sense to me. I don't know how exhaustive they were about looking at the wiring in general but they're pretty thorough on these things, particularly the mechanic who did the work. Also, there was a leak, a very bad one on the old aftermarket headers which were badly cracked on both sides, so new aftermarket units went in this year. What I'd like to know is why the CEL seems to be tripped by jarring to the underside of the car.

We had constant CEL codes on out Mini Cooper. I chased this by changing O2 sensors and it did not resolve, until I found that the mini originally came with NTK o2 sensors and did not like the new Bosch sensors. Once the NTK were installed, the codes have never come back.

Just my .002 to your problem.

Look at the wiring from sensors to DME. I suspect a mouse may have chewed his way through a bulkhead pass thru and you have intermittent contact with ground.