The problem is certainely the internal pull-ups. too weak to be good. try lower value let's say 1K.

BTW it's always agood practice to pca something betwen a PIC and the exterior world if you plan to use long distance cables. Opto coupler are still a good choice.

RB6:rB7 are still programming pins... look at the programming sequence of your pic... it make me think it could be something around it or not.

How about your MCLR pin?

How about the whole setup.

Bad ground screening, bad proto-board may give you some serious headache... worst @20MHZ. You may try to remove (for testing purpose only) those capacitor around your crystal.