fat guy on a bike....

Yeah, check the tires, rim tape, and rims for any sharp edges.

I started tri's at 255 or so and never had a problem when running at 120psi.

It's not a weight issue IMO.

Do you make sure you are running 120 psi before EVERY ride?  I will lost 5 to 10 psi over a couple days....  I check pressure before I go out, every time.

Is it the same tire?  If so, it could be a small tear in the tire that lets the tube poke out and burst.

I would say it's not because of your weight because I am close to your weight and haven't had a flat in forever (note to self carry that extra tube).  Slime is good for small punctures but pinch flats are something different. 

1.  Take a cotton ball and run it inside the tire.  If it snags on something find out what it is.  Sometimes tiny pieces of glass or whatever get embedded in the tire.  Slime would likely prevent this.  If you want to go slime just buy the slime tubes because it's way easier.   

2.  Do you mount your tires the exact same way every time?  It's easiest if you mount the label at the valve stem so you can determine if there is consistency in your flats.  How do you mount them?  I would wonder if this isn't just a mounting problem where you are pinching the tube between the tire and the sidewall (just a tiny bit) and eventually it just pops which is somewhat of a pinch flat.  I did this once and when it exploded in my living room I learned a valuable lesson. 

3.  It could be the rim tape but it all depends on where the hole in the tube ends up.  If it's on the outside then it's not that.  Rim tape is cheap compared to C02 and tubes.  Feel free to change it. 

There are a lot of potential problems but I think my suggestions should give you a pretty good head start on flat free riding.  Let me suggest Specialized Armadillo's as tires of choice should you desire new ones.     

Is it possible that your tube is not set inside the tire and wheel frame properly?  Perhaps it's being pinched?  I really don't think it's your weight; I rode at 340+ lbs and never got a flat. 

 I do use "toughies" inside my tires; always have.

Good luck to you... flats are no fun

Just out of curiosity, has anyone tried to foam fill a tire on a bike?

I have done this in the past with farm equipment and stuff that is prone to flats. They basically fill your tube with a two part foam that expands and acts just like air. When your tires wear out you just cut them off and do it over again.

 I don't know if it will work with the pressures required but it seems like a good solution.