It will dry out after a while. I would say that after 4.5 years it wouldn't hurt to change it.
Does your mobo chipset have a fan and heatsink? It would probably be a good idea to change the paste on that also.
Newegg.com - Tuniq TX-2 Cooling Thermal Compound
The system can fail to start at your OC settings because of your memory also. You should run Memtest86+
to test your memory, let it run through the test at least twice.
At higher voltage setting electrical component will degrade overtime, the average lifespan of a cpu is about 10 years. If you've been using it everyday, + or - a few days, for 4.5 years it could be starting to degrade. I would try lowering the overclock a little, and it would probably be a good time to start planning on a new system.