Hi,
inserting pull-ups 4k7 to 3V3 does not makes it really better. It takes a bit longer until the programm stops but it is not stable.
Do you know a combined 6-DOF sensor (like: SparkFun 6 Degrees of Freedom IMU Digital Combo Board - ITG3200/ADXL345 - SEN-10121 - SparkFun Electronics) which operates at 5V with I2C or with an PWM output for the single values? I could only find analog ones or some which operates at 3.3V with I2C. Are there also sensors with PWM output?