My problem isn't so much not thinking of something to say but I have the old problem where I say the bits I mean to say quietly loudly, and the loud bits quietly. For example, my boss goes 'Could you just give Caroline a push, she's struggling again' and I'd go 'Yeah that's ok, I'll give her a push....OFF A CLIFF' or I'd be 'mumble, mumble, mumble...YOU CLEARING IN THE WOODS'. I can't help it sometimes, it's like I start a sentence calm and get pissed off by the end of it.