Program hangs after adding floating point

I've found that when I add a floating point calculation to a program which previously worked it stops working after a few seconds. I've simplified my program to a timer interrupt with an ISR that increments a variable. The rest of my program displays (using printf) the value of this variable converted to seconds every so often. If I change the program (not the ISR) to recalculate the value displayed as a double it no longer runs properly. It will run for a few seconds before hanging. Does anyone know what could be causing this and how to fix it?

I am using Nios II Dev Kit 1.1 with Quartus 5.0. I have found the (apparently) same problem with both the legacy SDK and using the IDE + HAL.