Hello,
With all due respect to Brian T. I have no problems whatsoever with the 3 leg resonators, and use 20 mhz almost exclusively. Where I have had problems is from old breadboards. The higher the Frequency, the more affected the circuit is by corrosion, moisture, long lead length induced problems, and just plain funk. If you have to use longish wires to the resonator, then twist them, it seems to help. Make sure your power supply is up to the job and do not forget to tie up the MCLR and tie down any loose inputs. Do not run wires in parallel with the ones going to the resonator, as the added capacitance may load down the OSC.