Mmmmm.... I've investigated more and now I can say that it's not when writing TO CCON that my problem occurs. This seems to be an interrupt problem.

I've disabled the CCF bit in CMOD and then I can run the PCA with CR=1. It's not so bad !

Then, my UART routine doesn't operate correctly. Note that my UART routine is interrupt driven.

By looking the PCA datasheet, I see that if ECF is set, an interrupt is generated (CF flag) when the PCA overflows.

I've de-activated the PCA interrupt routine because with nested comments because I was thinking that the interrupt code was bad and was disturbing the processor timing, but the behaviour is still the same after. So this seems to be the interrupt transition which may alter the software. Could it be a priority issue ?
Currently, ESO=0 which means Serial Port priority low. But even if it's high, I don't understand why the simple fact that a PCA interrupt is set would disturb the UART routine ...

Any comment ?

Stephane