2006 Jeep Grand Cherokee does not start about once every 2 weeks. Battery, transmission and starter all all fine as well as all sensors checking out fine. NO CLUE on the problem - help!

My 2066 Jeep Grand Cherokee does not want to start about every 1.5-2 weeks in the
last 2 months.  First thought it was the starter and had a new one installed - had the
same issue and then had a second starter installed - same issue, but thought it may
have been the fact that the spade connector was not properly connected - fixed that
and they most recently, the car would not start again to where a THIRD starter was
installed.  Mechanic said battery, transmission and now starter all check out fine.  
Today he checked the computer to test any bad sensors and nothing picked up as
being wrong.  He thinks it's an electrical problem, but cannot seem to diagnose what
the issue is.  Last year, I had brought my car back to the dealer and they were
USELESS. I'm at a loss as no one can figure out what is wrong with my Jeep!