My buddy had this problem (with a Revo) a few weeks ago, but I kinda thought he was crazy.
I don't recall seeing a thread about it, so I decided I would post one up.
Here in FL, we have a lot of what is called "sugar sand". Basically it's super fine sand, and it's pretty much EVERYWHERE.
At any rate, I noticed yesterday my tires (Slash) are FULL of sand making them hard as a ROCK!!! It must get in via the vent holes, and then it just fills up the tire until it becomes solid.
This presents a couple of issues, it throws the tire weight off like crazy, adds weight, and it causes the tire to wear harder.
I wound up boiling them all four off, and I would bet there was a good half ounce of sand it each tire. It was also not doing the foams any good.
So, basically, if you have been running in fine sand, you might want to check your tires...
From what I understand the Revo's are even worse because they have larger vents or something on the back side.
My friend tried to silicone his vents, and I don't know if this worked on his Revo, but I am going to try it on my Slash, and I will just poke a few holes in the tire with a sewing needle to allow for venting.
I would have taken photos, but it was late and I was tired. I just wanted to get it back together so I could possibly run it tonight or hopefully this weekend.
The good news is that while I had the tires off, I painted my wheels just because I could.