Cheers Henrik,

10Mhz xtal, OSC set to 40 with HS PPL in the config and it works fine. I thought I was possibly on the right track, but didn't realise that I was in effect overclocking the PIC using the 12 and 20 Mhz crystals.

Now to look for a cheap I2C humidity sensor. The DTH11 that I was playing with uses pause statements which slow the rest of the code down.