Having fun

with new(to me) 2002 Boxster S Tiptronic, and at this moment I use 'fun' loosely

. I have been fighting various P codes having to do with Air intake and such. I fix one and then another that has been masked pops up, anyway I digress.
Just last week I started getting P1704 Kick-down switch and it won't go away. Now getting P1603 CAN timeout PSM not sending information. I have been using Durametric software and it's been well worth the $$'s, able to see many things that need looking at. The car has 132000+ miles but body and interior in excellent shape, also got it for reasonable price. It's getting a little frustrating since I had little to no trouble with my '84 TL for the five years I owned it, but again I digress.
I haven't been able to find any information on those two codes. I should add that the shifting has become a little erratic. I can put it in 'Manual' mode and shift but when in 'Auto' it will not shift up properly all the time.
I have been reading about changing the Oil, since I don't know if it's ever been done and the mileage indicates it probably should be changed.
Any help would be greatly appreciated.
Thanks,