I've been running Minecraft on Ubuntu for a couple days but I am often encountering the same problem. I started playing from the browser but after about 15 minutes it would freeze or lag so bad that I could not continue. I then learned to launch it from command line and continued to have the same problem. With the command line still open it gave me this warning:
"WARNING: Can't keep up! Did the system time change, or is the server overloaded?"
Figuring it may be a memory allocation issue, as was mentioned in another question, I tried the launch script provided in the answers there, which was
java -Xmx2096M -Xms1048M -cp minecraft.jar net.minecraft.LauncherFrame
This did not solve my problem, the warning is still occurring and I even had a little more trouble getting it to open with this one.This computer has run Minecraft before, when it had Windows, so I think the machine itself is powerful enough to do it. Oh, and I'm running Open JDK 7 for java.
Suggestions on how I can get this up and running for longer than 15 minutes?