Search the forum. Seriously. We have been over it a million times. A single temporal effect can cause this behavior. A single non-threaded Codec can cause this behavior. Simple transfer bandwidth exhaustion can cause it. GPU-accelerated features can cause it. whatever. You have not provided nearly enough info to even begin to guess and your statement is vague. In any case. It is probably perfectly logical and you will have to live with it.
What Mylenium said. MP works effectively within a very narrow of parameters. Until everyone that writes code for video and image compression/decompression, creates a plug-in for AE, or authors any program you may choose to upload to your computer gets fully on the optimized for multi processor processing bandwagon MP is going to break if you throw one think in the system that is not compatible. We are a long time from being able to utilize every bit of power available in a processor and every bit of memory in a system is going to be able to be harnessed for processing a frame of video that is less than 40MB in size. (a full frame of 4K is 36.5 MB in size) Ask yourself how fast the buss would have to be to push enough 40GB frames through 8 GB of ram per clock cycle to saturate 8 64 bit cores on every clock cycle, and even that would only utilize half of your systems power.