Quote Originally Posted by masosi View Post
I've tried with and without a resistor on the Rx and Tx Pin, there's no difference.

Does it make sense that the serial port seems to be sucking the life out of the circuit...?
No, but it makes sense that without the resistor, you probably smoked all of your ports.

But...back to the timing thing. The LED blinks, ok... Back to basics...
Do you have an LCD? Make a counter, one second, power up the PIC, start your stopwatch. Wait for 100 seconds. If your watch and the PIC/LCD match up after 100 seconds, you can be reasonably sure you're within 1-2% of the 4Mhz clock speed. The longer you go, the more accurately you can count it up.