You obviously don't know what kind of a website this is. We never suggest you take PCs to a repair shop unless it's absolutely warranted (like dangerous situations that only they should touch... they have life insurance policies for their employees!
Lag spikes can have several points of origin. It can be an issue with heat, so be sure the system isn't overheating. An OEM system will ideally have a CPU that hits no more than 65-70C under full load, and a GPU, depending on family, will do about 60-90C (both of these can vary).
Secondly, make sure your RAM is fine, grab a copy of hirensbootcd.org/download and burn it to CD or make a bootable UFD, then boot to the disc/UFD and run Memtest x86 for a good bit (at least two passes, which it'll tell you) If the RAM tests fine, move on.
Using the same Hirens CD/UFD, go to DOS tools, then hard disk tools, then on the third menu (#8 twice) go to GWSCAN (should be #5) and run a full test on EACH of your drives. If they pass the tests, then move on. If not, you may have isolated a possible failure point, especially if it's running RAID 0.
Beyond that, it could simply be a piece of hardware that is failing, like an out of date power supply, or aging components. Those two tests are a minimum for what I'd consider to test on the system first.