A couple of weeks ago my Mac Pro (Early 2008) started failing to wake properly from sleep – basically it would boot from scratch rather than awaken.
PRAM zapped and all the usual stuff you see written across various forum. Then at the point of installing a replacement backup battery, I noticed a bright red light on one of the RAM risers.
Cutting a long story short, one of the RAM modules has failed. Checking the system profile, 4Gb of installed RAM is missing. The mac has wisely ignored the failing bank of RAM, but it would have been nicer if it had also alerted me to the fact prior to looking inside.
End result: physically removed the modules on the failing bank and now sleep/wakeup works as normal again. Fingers crossed the memory supplier will replace the failing package on their lifetime warranty.
(Update: August 2018, I should have updated this at the time, but Crucial sent a replacement module very quickly and that has worked without fault ever since – the machine is still in regular use at time of writing)