From what I've been reading in the past 10 minutes in order to try and give you a specific answer, it's not so much one temperature but rather high-lead solder (which apparently is what Nvidia was using) is prone to cyclic failure, that is it weakens over time with large temperature changes. Laptops typically get powered up/down many more times than a desktop part, so this makes them more susceptible in general to such stresses. Increased fan output can help, but from what I've read, many believe that will only delay the problem, perhaps outside a warranty period so manufacturers don't have to take responsibility for the problem. Too bad the chips are likely surface-mount or one could resolder the chips relatively easily with higher grade stuff and then forget about it.
What I don't understand is why Nvidia, once they discovered the root of the problem, didn't immediately simply switch the solder they were using to lower lead or no-load solder and then newer batches would be without the problem (some comments from Nvidia seem to indicate they DID do that, but 3rd party articles poo poo everything Nvidia says and declares them ALL faulty, suggesting Nvidia preferred to HIDE the problem and keep on turning out bad batches. The question becomes which is true?) If the problem is in all the batches, Nvidia SHOULD be forced to offer higher quality solder replacements for all units, IMO. Extending a warranty will not fix the root problem. If, however, some batches did have better solder, they should release the information about which batches contain the fix. Otherwise, I would assume they're all bad since it makes no sense for them to hide the good batches.