Make it STOP!!!

What to do? My 4 year old has decided that it is more fun to use his underwear as the toilet versus the toilet itself. Now, I normally would not have a problem with this-that is if it only happened a few times and was accidental. However, this little boy has been doing it for the last three months-at least. Now, I know accidents happen-he's playing outside and forgets, but I'm finding I don't think this is the case-entirely. He will be sitting in front of the tv and decided to go, he will be sitting in the car and decided to go, or as previous mentioned be playing outside and decide to go. It usually only happens in the afternoon and I have been trying to consciously remind him. I even asked the Dr. about it, but I don't know why he is regressing and it's making me a little impatient-okay upset. Rand regressed for a few weeks, but that was a little while after he had just been trained. Mont is going on a year of being potty trained. It's as if he doesn't really care. I'm kind of at a loss and don't know what to do-I feel like I have tried every tactic in the book-bribery, being a nice mom about it when he does it, being the mean mom, giving cold showers, putting him back into diapers (pull-ups-since he wears them to bed). Am I missing something...I just know I'm getting tired of it and I know he is getting tired of me being on his case. How do I make it stop?