Start with testing your RAM. FBDIMMs fail. So that is one thing to watch.
Disk drives need to be zeroed.
A clean install and leave it at that.
You sound like directory is corrupt (again) and need to repair it, need to use Safe Boot at the least, and avoid your old apps and maybe you have some corrupt files and prefs in your home folder.
Those hard forced restarts mean the directory no matter what it was before is now in worse shape.
SSD make sure they have latest firmware.
Mine is 2006 w/ 8GB and 8800GT plus SSD and 10K WD VR plus WD black data and WD Green for backup. and it is only 2.0GHz and it runs fine.
Monitoring your temps? how old are the FBDIMMs?
Do you own Hardware Temperature?
What about SMART Utility, seems to be the best at detecting and mapping out bad sectors (or WD Lifeguard under Windows).
Apple Hardware Test: one that if it does not have errors does not mean there are none, only none that it tests for which excludes a number of things.
And graphic cards. The 7300GT, the X1900 and 8800GT are all to be suspect and you can't really monitor GPU temps as far as I am aware under OS X.
Dust. It clings to RAM, the X1900 is a dust magnet and clogs up.
Running the system fans (3 of 4) should be a given and must to increase the base rpm to around 800-900 rpm to improve air flow and cooling.
Amazon has 2x2GB FBDIMM kit for $28.95, take advantage and try a set if it seems like RAM issue.
Especially if you have had problems with an SSD in the past, it is very important to deal with deleted data on the SSD. This can be done by periodically Zeroing the device and restoring what was on it, or by adding third party TRIM Enabler or equivalent.
Apple does not endorse TRIM Enabler, but many here have used it and it seems to work as advertised. When you run Disk Utility ( Repair disk ) with TRIM Enabler in place, you get another line of output and it takes an additonal minute, "TRIM-ing unused blocks".