I have 2 GTX 660 that work fine when run on their own, but when I try to run SLI the game I try to play play a game will most likely end up in the computer restarting or sometimes just freezing the game.  The bridge seems to be working because my computer is recognizing it. I re-installed the drivers but that didn't help. My next guess is the power supply, I am using a NEX 750G. Is this enough and if so are there any suggestions on what else could be wrong?

That did not appear to help.

EVGA  Supernova NEX 750G

Unfortunately I don't have access to another system that can run SLI. Something I have noticed is that if I let the computer sit for a while after turning it on it seems to run better. If it isn't obvious already I am not very well versed in computers, but could this be an indicator of something or just a coincidence?

Sounds like a PSU issue..If you have the 750 G1 that has four +12V rails, VGA1 and VGA2 are on the +12V2 rail while VGA3 and VGA4 are on the +12V4 rail..If you have your GTX 660 PEG connectors (6/8-pin supplemental power connectors) plugged into VGA1 and VGA2, try moving one of them to VGA3 or VGA4 which will take some load off of +12V2 rail and redistribute it to the +12V4 rail..The flip side would be you have the PEG connectors on VGA3 and VGA4 and you'd move one of them to VGA1 or VGA2..If it works okay, then you need to RMA your faulty 750 G1 PSU..If it doesn't matter, you still could have a faulty PSU.
 
If you have the 750 G2 with a single +12V rail or if redistributing the load on the G1 don't matter, you could hotwire (see Step 4) another PSU (if you have one or can borrow one) and use it only to power the PEG connectors of one GTX 660 while your installed PSU runs the other GTX 660 and the rest of your system..If it works okay, you need to RMA your faulty PSU..If you cannot (or don't want to) do this, then the only alternatives would be to swap out your PSU with a known good one of sufficient power or take your system to a repair shop for SLI and PSU load testing.
 
You could also try reducing your GPU core clocks as far as possible using EVGA Precision or MSI Afterburner to see if reducing the load that way helps any.
 
If you need a good idea of your overall system wattage requirement, use this PSU calculator. 

bourniboy
Reducing the core clock has helped for the moment. I dont know if this is a permanent solution or just a coincidence.

Reducing the clocks point to a faulty GPU. I would suggest testing the cards in SLI in another system running the cards at default clocks to confirm. 

bourniboy
 
Could it also suggest that the power supply isn't enough?
 

It's possible, that is why you need to test the cards in another system.

I took the computer in to a local shop and they could find no issues.  Both gpus look fine and I tried a new SLI bridge but nothing changed.