I have friend with a 2005 LSJ that has been going into limp mode after WOT, and recently at random times while crusing. It is throwing p0603, p0315, and p0300. Mods are 2.8, 60s, fujita intake, header/dp, ported m62, ls4 tb, with a zzp canned tune. We are having a hard time diagnosing the problem, can't find any vacuum leaks or looose connections so any help would be appreciated.

 

I know that you said you checked connections, but check the PCM connectors,
These,

It might save you a big headache ask me how I know

 

What he said. And dont just stare at them....undo them and clean the connections