MAP sensor, FPR or (if he's running one) fuel pump resistor. MAP sensor is the first thing I'd check and then I'd reset and reupload the tune he's using from a backup to make sure nothing's just screwy in the MS' memory. All you need to test the MAP sensor is a multimeter and a reasonably accurate vacuum pump.
If the tubes you're referring to are the exhaust manifold runners, well, the temp drop could be accounted for by the excess fuel preventing a full burn (if it's not completely blowing out the spark as it is). Sticking valves aren't terribly common on these engines.... but they're not too hard to check for so feel free, I doubt that's the issue though.
__________________
If something breaks or you need to contact a member of the administration please post HERE. Unless it's a private or administration matter please post it on the forum. It benefits no one else if car related questions aren't posted for future users and takes away from the time I'm able to spend helping on the rest of the forum.
If you're so inclined I'm always more than happy to accept tips via PayPal. 
Tip Jar ---> 
|