2003 Silverado electrical issues

I am having issues with my 2003 Silverado. it started out with the abs and
parking brake light flashing with the audible chime. this happened briefly
and intermittently. it progressed from there to my radio cutting off and on,
my gas gauge, oil pressure, voltage, etc. dropping out, then coming back
on. then progressed to the amber theft light coming on along with the
airbag light. I have put a new alternator, ignition solenoid, ignition switch,
and also switched the instrument cluster out to see if that was the
problem(local gm service said that was the issue...didn't work). Also, from
time to time, my transmission wont shift correctly. Had the transmission
serviced and the downshift module replaced...it still happens from time to
time. there really isn't a rhyme or reason that is consistent, just seems to
happen when its warmer outside. I have had a certified gm guy look at it,
but he's at a loss because it wont replicate these conditions while hes
looking at it. he sees several codes (loss of communication and voltage
codes) but needs to see it act up while he has his computer on it. Any
ideas?