Need help diagnosing excessive memory usage on my blog

About two or three times a day, my website uses too many resources, causing a fault and sometimes 508 errors as it goes offline briefly. I just discovered this a week ago and talked with my host, and the best they can tell me is it’s probably a plugin, but I am having a difficult time diagnosing the issue.

So far I’ve removed all the plugins I think I can do without, cleared my Limit Login Attempts log, and removed Bulletproof Security because I thought it might be using a lot of memory. I also got Quick Cache to try to help resource usage, reduced the number of blog snippets on my presentation page, and reduced the number of pictures in the blog slideshow. Nothing has helped.

What I’ve noticed is that after I make a change, the usage limits shown out on my webhost’s cpanel go down, and I think I’ve made an improvement. Then I leave for the day, come back, check stats and my site has gone well over 100% CPU and input/output usage three times in eight hours, and I don’t know why.

I use Parabola as my theme and only get 150-300 hits per day. It’s a low-traffic blog. (I’d rather not post my URL; I got a DDoS the last two times I posted my webpage to these help forums, which may be coincidence, but I don’t want to tempt fate. If you absolutely NEED my URL I will post it, though.)

My current plug-ins:

Akismet
Collapsing Archives
Configurable Tag Cloud
Jetpack
Limit Login Attempts
Official StatCounter Plugin
Quick Cache
Quick Page/Post Redirect Plugin
Wordpress Firewall 2
WP Overview (lite)

All are activated and in use.

Any help you can give would be most appreciated!