The external crystal requires a startup time to stabilize. If you try to use it before it is stabilized when using the PLL at 24 MHz, the output of the PLL may exceed 24 MHz and the maximum specification for the program memory, causing corrupted reads and a crashed program. To properly use the PLL, the user must allow the external crystal to stabilize and then turn on the PLL while the processor is set to 3 MHz. After the PLL is given time to Lock with the external crystal, you may set the processor clock to any value up to 24 MHz. The timing specifications for the stabilization times for ECO and PLL are given in the datasheet. They are 500mS and 10mS respectively for the ECO and PLL.
PSoC Designer version 3.0 and above includes code in the boot.tpl file for proper stabilization time of the ECO and the PLL.