Glad that worked, noop. You might want to try it as well, AB. In case you're wondering, here's a really basic description of what actually happened when you moved that slider.
The problem you were having was that your computer was trying to render a lot of sound on the fly, almost in real time, which can be difficult on the system if you're trying to make a lot of different sounds at once (or really complex ones). So, by increasing the buffer length, you're telling the computer not only to render what you can hear immeadiately, but it also renders ahead a second or two ahead to 'lighten the load', so to speak.
Now, I'm not feeling particularly articulate so I'm going to quote a dictionary on the definition of a buffer, just to make sure you understand:
"(computer science) a part of RAM used for temporary storage of data that is waiting to be sent to a device; used to compensate for differences in the rate of flow of data between components of a computer system"
So, in short, its not a problem with your song so much as your computer having a hard time keeping up. Don't worry about keeping the buffer size high, though. I always have mine maxed out, and I know others have similar experience, too.
As for the "disable max poly" thing, here's a simple explanation. Your fruity instruments have Polyphony settings under their MISC tab (miscellaneous settings). In there is a number, default 32 I believe, that you can drag higher or lower. A polyphony of 32 means that instrument can only play 32 notes at a time. If you drag it down to, say, 3, then that fruity instrument can only play 3 notes at a time. If there are more than 3 (or 32) notes, then it will start cutting off other notes short in order to play the new ones. This can be advantageous for slower machines that can't handle playing many notes at once (or for creating an unusual sound effect).
So, when you check the "Disable Max Poly" box in the rendering settings, FL Studio will render the song as though the polyphony was set so high that it could accomodate every single note. Or, more accurately, it disables the maximum polyphony limit, so that every note can last its entire duration.
To summarize, polyphony = how many notes can be played. Disabling the polyphony = an infinite amount of notes could be played.
However, as to why turning the "disable max polyphony" off (effectively -enabling- the max polyphony) would mean you can suddenly hear new notes I can't imagine. I'd probably have to see the .flp to tell for sure O.o
Anyways, hope this helps!