-Get rid of that 9v battery and make up another battery pack. Get a bunch of NiMH AA's and make another battery pack, anything but that stupid 9v battery.

-Put decoupling cap's across Vdd and Vss on the PIC.

-Turn off the brown out detect on the PIC in the config fuse options.

-4Mhz osc - accuracy isn't going to help here. As long as it runs, it should be good.

-USB programmer from eBay - spend the money and buy a PICKIT2. You'll be happier in the long run.

-One power supply for the PIC, one power supply for the motors - good idea.... Separate grounds, not a good idea. Connect the grounds together.

-Put low value inline resistors on the lines between the PIC and the L293D.