I remember a cartoon, I think, with a line where one of the character says, "I mostly.." something and then they pause and say "mostly." again. I've looked for it and it's driving me mad! Please someone help me! What is this from!?

Aliens. Newt. "They mostly come at night. Mostly." Or something to that effect.
edit: More accurately its: "We'd better get back, 'cause it'll be dark soon, and they mostly come at night... mostly."

One episode of South Park had Cartman see that part of Aliens. For the rest of the episode he uses the "it mostly" x'es "at" y ", mostly". So that is probably what you are thinking of.

I love you, thank you!

The South Park episode is available online.