I supposed it would be optifine, as the XyTanks use a special effect to show the tanks' boundaries (the glowing blue lines, these are a -hardcoded- feature from XyCraft, e.g., there is no 'glowy blue line texture' that could be the problem) and I thought this might cause OF to hiccup. Other than what Zica and southern said, another similar thing to try would be to 'start' with all OF-settings turned off (AntiAliasing, DoF, Fogcolors, Particles and whatever the heck else there is to turn off^^) and see if that works. If it does, turn on one setting after the other - 'cause if it DOES work when everything is set to minimum, there MUST be one (maybe two) settings, that cause this problem to occur.