How is my PHP site wasting "cgi resources"?

According to my host (Netfirms) the reason that my site keeps timing out or being “down” randomly for several minutes is that “I’m using too much cgi resources”. At a peak of 20,000 visitors in one day I had images replaced with the message “CGI LIMIT REACHED”. (Normally it’s about 3,000 to 5,000 visitors, which simply causes long load times or time outs about half the freaking time)

Now, my site uses lots of “includes”, and also URL variables, if statements, and so forth. I also have lots of large Flash games/movies, but even so, I’m only using about 300GB/month out of my allotted 3,000,000GB/month… so at this rate, it would be IMPOSSIBLE to actually use the bandwidth I’m paying for (since it’s crapping out at about 0.00015%).

My limits are here: http://flyborg.netfirms.com/cgilimit.htm
(scan down to the “advantage” plan)

I’m not super experienced with PHP (I mostly use Flash) so I have NO IDEA what could be wasting resources. Any help?
IE: Is it CPU time? Processes? What kind of PHP code can waste resources? I use lots of “includes”, if that helps.