If your resource pack has custom fonts and you have optifine installed, that might be the issue. Optifine has (and had) a few issues with HD fonts, removing them from the resource pack might fix the problem.
Otherwise, install this mod (drop it into your mods folder). Then, whenever you start Minecraft, another small window will open which has a button labled Create Thread Dump
. Do whatever it takes to get it to freeze and wait a minute while it's frozen, then press that button. Copy the thread dump from the text area above the button to some pastebin site. Then post the link here.
Create Thread Dump
. Do whatever it takes to get it to freeze and wait a minute while it's frozen, then press that button. Copy the thread dump from the text area above the button to some pastebin site. Then post the link here.