Optifine causes problems on some computers.
Make a backup of modpack.jar (located in %appdata%\.techniclauncher\tekkit\bin in Windows XP, not sure about other OS).
Open modpack.jar with Winrar or 7zip.
Delete the following files:
aaw.class
acq.class
adz.class
amd.class
Config$1.class
Config.class
ConnectedProperties.class
ConnectedTextures.class
ct.class
ctm.png
CustomAnimation.class
CustomAnimationFrame.class
CustomColorizer.class
GuiAnimationSettingsOF.class
GuiDetailSettingsOF.class
GuiOtherSettingsOF.class
GuiPerformanceSettingsOF.class
GuiQualitySettingsOF.class
hu.class
ItemRendererHD.class
jy.class
l.class
lr.class
lv.class
mx.class
NaturalProperties.class
NaturalTextures.class
nl.class
nq.class
RandomMobs.class
Reflector.class
template.natural.properties
TextureHDCompassFX.class
TextureHDCustomFX.class
TextureHDFlamesFX.class
TextureHDFX.class
TextureHDLavaFlowFX.class
TextureHDLavaFX.class
TextureHDPortalFX.class
TextureHDWatchFX.class
TextureHDWaterFlowFX.class
TextureHDWaterFX.class
VersionCheckThread.class
vl.class
wc.class
WrUpdateState.class
zq.class
Download the latest Minecraft Forge for 1.2.5
Copy the following files to modpack.jar:
aaw.class
acq.class
adz.class
ct.class
l.class
lr.class
vl.class
This fully removes Optifine. If you still have problems after doing this, Optifine was not the cause of your problems.