I have an E-Commerce site, which gains a sudden peak of very high traffic when I turn on different ads campaigns.
From this moment on, my current dedicated server (Hostgator, 4GB Memoray, 100mbps uplink) becomes unresponsive.
I have worked too much on my site code's optimization including cache mechanism. But it seems to be a server issue.
Any guidelines what should I do?
I can't really answer the "who should I use" part of the question (as it's off-topic), but given that I do have significant experience in making sites/applications scale for high-traffic loads, I can definitely suggest that you look at getting a Reverse-Proxy CDN.
The last company I worked for used Yottaa for this, and were able to use their services to survive some absolutely huge traffic spikes generated by TV advertising.
There are lots of CDN services available, but you'll probably see the most benefit from one with an "origin pull" mechanism, where user's requests hit them first, and they request the page from you once, and then all further hits for that document are served by the CDN's servers.
You'll do best to pick a half dozen different CDN providers, and then set them off against each other, and see who can battle the best deal for your requirements. Big CDNs like Akamai are probably gonna be insanely expensive for your scale/needs.
I'd mostly forgotten that there's actually some Free CDN services from the Coral CDN, and Cloudflare (free to a point, I seem to recall).
One of the intriguing things that some CDNs can provide now is SSL acceleration, where they serve the secure portion of your site with a Subject Alternative Name certificate they generate, so that they can "pretend to be you" in terms of what the users see.
You could start by spending a couple of years learning Linux/Unix systems administration, another couple of years learning about programming and applying those skills, then spend 12-18 months learning about HTTP addministration, reading up on the topic, building test systems and experimenting with different configurations and load profiles.
Currently you're not even providing the most basic of information about what's happenning to your server which rather implies that you don't kbnow the basics of how to start addressing the problem. There are lots of stackexchange sites covering a range of topics, however you're unlikely to find any with questions like 'How do I perform brain surgery to remove a small tumour' or 'How do I fly a 747'. There's just too much to cover in the question nevermind any answers.
It's great that you're getting enough traffic to hose your server - particularly if it's an e-commerce application. That should provide sufficient justification to go out and pay for some quality support with solving your performance problems.
