input of midi is delayed, but only in certain projects.. what is this? and, how do i fix it?

I get to a point in my projects where i've input a few instruments with no problems at all.

Then, maybe after pluging in a few Waves plug-ins and getting a bit more demanding on cpu power my INPUT midi becomes delayed...

Not the playback, but the actual playing of the instrument... This is a huge problem because as soon as you're about to finish up a song

all of the sudden my input method is totally screwed! -- I've had to resort to the pencil a few times!! What a pain!