@bromatt I recently experienced a blown/broken crystal, which didn't behave as you describe - the circuit simply didn't function. However, I don't know if there's any weird behaviour before one fails completely.
Without seeing the circuit in action, it sounds like it might be resetting for some reason, such as a temporary load which affects the voltage. If I'm right, the variation in blinking is therefore to do with it restarting the whole program again. I've seen this happen in unprotected circuits controlling motors, and it may reveal latent issues with having some contacts which are not conducting well for some reason (perhaps corrosion in the breadboard connections).
Protection from variations in load is one primary reason, as well as stable analog reference voltages, for the differences between the minimal programmable circuit at...
http://shrimping.it/blog/shrimp/shrimpduino_bb_minimal/
...and the more practical, protected circuit at...
http://shrimping.it/blog/shrimp/shrimpduino_bb/
...which is recommended for any substantial physical computing work.
Try the more protected circuit instead, and be sure everything is cleanly contacting.