We used to believe that heat and vibration took the top spots, but have since come to the realization that low supply side voltage should take the top spot.
Yes, it doesn't take a genious to determine the inherent design issues with placing these modules on the top of an engine that gets pretty warm and vibrates continuously. What's not so clear, however, is that each morning you start your truck it is essentially running only on batteries until your glow plug circuit has completed since the alternator is not able to keep up with the 230 amp load associated with the glow plugs and the base motor draw. See items 13 and 14 on our Maintenance Tips page for more information.
Of course, this wreaks havoc on a module already asked to take the nominal 12 volts of supply voltage and convert it to the 45-50 volts expected.
On that front, recognize the need to maintain fully charged batteries - batteries with a float voltage of 12.6 or higher that don't dip terribly for the first few minutes of engine run time. While it's common to have people report that their batteries' voltage dips down to 11.4 or so during this time, it's these same people that have their FICM's fail and are left to wonder why.
An excellent primer on batteries can be found at Battery Stuff.com.
For the vibration, ensure that the grommets holding the mounting brackets to the FICM are still soft and pliable. If they aren't, get replacements. It might be a good idea to get different ones just regardless.