WordPress & Server Load (Spam?)

2 of my clients website are causing problems on my server.. both of them WordPress. I have busier non-WordPress sites that cause no issues at all.

At random times server load increases so much that the server becomes unresponsive and every time logs show a lot of traffic going to both WordPress accounts. I have vBulletin huge forums and online stores that are much busier and don’t cause any problems.

Both instances of WordPress don’t use many plugins if any at all, they are kept up to date and both are pretty small websites but do receive a high amount of traffic.

i’ve been forced to execute a cron every minute that restarts apache if the load is beyond a certain point, eventually bringing it back to normal but this is still a couple of minutes of an unresponsive server.

My server is more than capable of handling the average load the websites ‘should’ bring.

One thing i did notice is that it was happening at the same time on certain days, then i noticed one of them had over 1000 spam comments… i increased the spam protection which has reduced the amount of comments but i’m wondering if there is a automated spam program or bots attacking the website at such a high rate that it’s crippling the server?

The most recent case was 2am, when the website would technically be dead, so why is it reporting such a high amount of traffic? it can only be spam i just don’t know how i can prevent this happening?

Any suggestions?