Spinning beach ball after insert when returning to sequence

I am editing AVID MediaComposer on a MacBook Pro with LaCie thunderbolt drive. I was editing on 6.0 but a few days ago, I encountered a problem that every time I placed an edit in the sequence, or trimmed, a beach ball would appear when I tried to play my sequence to review my edits. This :20 delay is driving me crazy. I upgraded to 6.0.1.1 last night but the problem still exists. I also transcoded my sequenece to the HD, thinking that it might speed up the project. Nope. Any ideas? I would love to enjoy the 4th of July, but instead I am watching beachballs on my laptop.
Ali