Reselecting the texturepack ingame sets FPS to 15 fps

[v01ddd]

Launcher Version: 1.0.1.3

Operating System: Windows 7 64 Bit

Java Version: Java 7 Update 5 Build 1.7.0_05-b05

Antivirus Program: Bitdefender

Description of Problem:

Whenever I am ingame on a map (not in the main menu), go into the graphic options and select a new texturepack, the FPS drop down from 50 FPS to 15 FPS. This even happens when I reload the map, or when I reselect the same texturpack that worked before. This map is then stuck on the 15 FPS, no matter how low I put the other video settings. additional info: I use the standard texture pack and sphaxpurebdcraft 128x128

But if I create a new map with the same texturepack and high video settings the new map can be played at about 50 FPS again.

Error Messages:

none, game doesn't crash

Error Log:

no error log as the game does not crash