Apache Down With 503 Error / Http Requests Get Redirected / Is Server Hacked

My Linux Server's Http Daemon (Apache) would stop serving websites ever so often, as soon as apache is restarted the error fixes iteself only to resurface within few hours.

The apache process would still be running i.e. apache does not die but no websites hosted on my server would be accessible from browser. And when this happens the apache logs do not log any http requests.

Instead when this happens all http requests to my server would be redirected to some weird Trojan website and my Norton Antivirus would show an Alert/Warning, for example;
"Browser exploit at www.xxx.xxx was blocked"
Risk Name: MSIE WebViewFolderIcon ActiveX Control BO

or another error like;
"Auto-Protect has detected Trojan.Fakeavalert".

At first i thought the problem could be with my Laptop/ISP so i logged on to the server via SSH and opened try to open a website using command line "lynx mywebsite.com" and it shows following error;
"Alert!: HTTP/1.0 503 Service Unavailable".

Now if i assume my laptop were to be infected, then as soon as i restart my apache and visit mywebsite.com eveything returns to normal with no such warnings. Why do i see those norton error messages only when apache is down with 503, and when apache is down with 503 how come the http requests always get redirected to some suspicious websites and nothing gets logged in apache error log?

I think my server is being attacked causing http to get unresponsive and thereafter http requests to my server are redirected to some malicious website, is this correct?

Also, i suspect this is a php script exploit as some customers have reported that google have blocked their website due to security reasons, i found <iframe> tage inserted in some php pages which i fixed.

Also, another thinh i noticed;
when apache responds with the 503 it is referencing PHP 5.1.4 in the header response:

[root@]# curl -I xxx.xxx.xxx.xxx (my server ip)
HTTP/1.0 503 Service Unavailable
Server: Apache
X-Powered-By: PHP/5.1.4
Retry-After: 20

I am running PHP 4.3.9m why does apache responds with PHP 5.1.4 when this 503 error surfaces?

Also, since my apache was dowan with 503 error a customer mailed in today saying;
"It seems that my site www.xxxx.com is regularly down, and the winlogon virus is involved."

I suspect this is again due to the fact that http requests start getting redirected?