Are you running 64bit OS? If you are, are you running 64bit Java? If not, I would suggest that.
There are some posts that say allocate more memory to Java, and there are others that say not to, as it may cause other issues. I have gone both ways, and havent seemed to have an issue.
You could install the Optifine mod and see if the settings in there would help.
First though I would try switching back to the default texture, and installing 64bit Java (if you have 64bit OS..)