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Cannot get C2000 core of F28M35H52C1 to run at 150MHz

We've been developing a product using the F28M35H52C1 micro. We've had numerous batches of PCAs assembled with the micro on them during development. All of them have been working as expected. Our latest batch of 10 boards, which is identical in design to the previous batch, has had problems.

3 of the 5 controllers we've powered up have had problems running code with the C2000 at full speed (150MHz). This is not due to my poor coding, it also happens with TI's basic dual core "Blinky" example.

Here is the code I'm using to set clock speeds:

    // Setup main clock tree for 75MHz - M3 and 150MHz - C28x
    SysCtlClockConfigSet(SYSCTL_SYSDIV_1 | SYSCTL_M3SSDIV_2 | SYSCTL_USE_PLL | (SYSCTL_SPLLIMULT_M & 0x0f));

We're using a 20MHz TCXO as an external clock source.

Any ideas? Do we have a bad batch of chips?