I don't think there's much particularly non credible about Cobo - from what I've read (here probably) he suffers with the illness of depression, and as a result his career was very up and down. So the picture of guy struggling with illness, but when he gets his head clear he can express his talent before dropping off (regardless of whether he's doping or not), seems a lot more credible to me than the alternative, which seems to be: guy dopes a bit, stops, dopes massively for one race, wins, then stops again. Which doesn't make much sense on the face of it.