Need more information when it does it.
Does it try to crank over and no start or does it not even crank over?
No cranking at all when it does it. No clicks, no turning, nothing. As if the starter, fuse or relay were removed completely.
Do you have cruise control?
Yes
If it does crank over, does the tach move (even just a little bit)?
If it does crank over and tach does move, it's fuel pump or fuel relay
If it cranks but tach doesn't move, its the CPS
N/A to these three questions since it doesn't crank at all. Good to know about the CPS for future reference though
If it doesnt even crank, it could be the starter, starter relay, battery or terminals
New starter was just installed. Putting in a new relay and cleaning the terminals and ground today. Will report back in a few days (since it's an intermittent issue I have to wait a couple of days to make sure the problem doesn't come back).
I don't think it matters, but my car is a base Boxster with a 996 3.4 swap and 996 ECU, just for reference (or showing off 
)
Thanks for the help 