"PF" is a Power Failure alert, as you probably have figured out by now. According to the owner's manual for the model you have listed, you need to press "OFF" on the control panel anytime "PF" appears to clear. The, reset the clock if required. If this does not correct the problem, you may have a defective control board.
These control boards are often suseptible to power surges and electrical storms. If you determine the board to be defective, the following link explains step-by-step how to replace one:
http://www.fixya.com/support/r3727366-replacing_control_kitchen_range
Replacement parts (if required) can be purchased at any of the following websites:
pcappliancerepair.com
appliancepartspros.com
searspartsdirect.com
repairclinic.com
Prices vary between these sites, so shop and compare. The first three sites listed also offer helpful exploded view diagrams to help you locate and properly identify the parts you need. Hopefully, this is just a simple operator intervention issue that does not require parts.
If you have any questions please post back and let me know. I hope you find this information helpful.