xpolior
As others have said we need more information to help you. Which ut version, which map, which renderer you use etc.
Without that info all anyone could really guess for you to do is to switch/update your renderer or the easier route to just patch to v469 if you haven't already done so. The 469 patch includes updated renderers so the hard work is done for you. Just copy your ut install folder somewhere else as a backup first for the unlikely event you want to go back to your old install.
