Problem solved. What I have learned: My interrupt routines are all written in C, so the compiler pushes all the registers onto the stack no matter what. This seemed really stupid to me, so since I had nothing in my main file, I took the repetive code from the Timer interrupt and place it in line with a delay routine in the main file. Everybody following so far? Alas, the problem went away. However, I have found that I still need to re-write my external interrupt in assembly so that I can tweak the pushes and pops. Thankyou to all for your assistance.