Alain

Just thinking aloud. I am assuming the 16f628 / 20 in the subject means you are thinking it runs at 20MHz or is it something else? The INTOSC for the 628 is 4MHz. Perhaps you have declared 20MHz for OSC like below. Is this a possible cause?

define OSC 20

I do remember having faced a problem like this once before, but unfortunately, I cannot recall what it was and in which project. It too had to do with an LED though. All I remember is that I had to write the port twice with a delay between the 2 writes to get it to behave.