After i added all the DLC's to FNV, i noticed i could only get about 15-20 into the game and then the PS3 would freeze up requiring a reset.
Prior to adding any DLC's, i could get about 3-4 hours until the PS3 would freeze up.
The solution i discovered (and may work for you) is to change the PS3 resolution to 480p.
Now i know the graphics will totally svck, but after multiple times of having to deal with the annoyance of the PS3 freezing every 15-20 minutes constantly and having to save progress every 5 minutes, i find this option is much better as prefer game play over graphics, i simply get used to the horrible graphics (been playing since PS1 days).
Im not going to speculate or investigate into any reason why this happens, although i could if i choose to.
After a brief investigation, lets just say it appears PS3 is getting a bit too warm.
As an additional test to confirm my theory, i took my stock PS3 and placed it inside a small 4 foot refrigerator @ 40 degrees F, i was able to play without any freezing for more than 2 hours @ 720p.
I wont be keeping the PS3 inside the refrigerator, its not practical at this point but perhaps if i get the willpower, i may choose to modify the PS3 for better cooling in the near future, for now, im too lazy.
This temporary fix may or may not work for you, it may depend on how you have your PS3 connected.
Mine is connected video & audio HDMI to a 32 inch LCD tv.