Hi Armando,

I see you have done everything exactly by the book.
It sound like you are correct about the serial cable adding a ground. Same with your finger on the LED.

What do you use for a power supply? Low noise? Is the ground connection between the supply and your PIC circuit good?

Do you have two capacitors near the power pins of the PIC to ground.
Something like a 10 microfarad. and a 0.1 microfarad, in parallel.

I would double check all the intended ground connections to see if some might be forgotten.

Maybe some of the unused / un-terminated pins are picking up noise.

Any other suggestions for Armando out there?

-Adam-