CPU Overloads will be the death of me

I know this issue has been addressed a thousand times over, yet here I am. I am getting ridiculous amounts of CPU-related overloads, even just in playback mode. I am running Steven Slate Drums, through a couple of buses for reverb/compression. I know this is a very CPU-intensive instrument, but this level of overloading just seems strange. Often, the overloads happen the second I press play. I don't recall having the issue in the past. Lately, it's been happening even if I'm just playing back an area of the project with no regions/audio files, even with my I/O buffer size at 1024. I'm running out of ideas. I'm using an iMac with the 2.7GHz i5 processor, on 4GB of RAM with a 1TB SATA disk. Is this just not enough RAM for the job? Any help is greatly appreciated, as it's getting to the point where I can't even play back my project with just one instrument running. Thanks.