Just found out - as I sort of expected - that the problem is in the x6/x12 clock programming.

Although according to bith FM and my parallel programmer the chip is set to x12, it is in actual fact running at a x6 clock.

I also tried to set the CKCON bits to achieve x12 clock compatibility for my timers and UART, but that doesn't seem to work either. This leaves me somewhat confused. Although it would be possible to change all timing parameters in software, I would rather leave the code unchanged and adapt the chip. Any suggestions ?