I'm guessing you meant 16F877A?

Try placing a 0.1uF cap between Vcc & GND, and locate the cap as close to the PIC as possible. That will probably fix it.

The A series are very sensitive to ESD, EMI, etc, and if you don't have a 0.1uF cap strapped across Vcc & GND close to the PIC you'll see this happen. Just getting near one or touching wires will make it go like splat.

If the cap doesn't cure it, look for bad or lose connections. Code looks OK.