Everytime when i adding "flaming burst" (and sometime "ice burst") game crashes. Exactly right this moment when add arrow pushing. Clear weapon or continues enchanting - doesn't matter. Flaming burst - 100% crash. GOG version. TFE-X + 8.1.0.NC. Windows XP. Maybe there is some fix for s*it like that?
Most likely you are trying to surpass the maximum allowed "enchant points" that can be put onto a weapon. The engine has a (last I heard) hard coded limit to the number of total bonuses that includes stuff like +1, +2, +3 and all other flags. Flags have various point costs as well. Try enchanting a +1 weapon with flaming burst and I assume it will be fine.
What kind of crash please? Game become non-responsive? Crash to Desktop? or something else. When exceeding the enchantment limit the game should just refuse to do it. How new is the problem?
Do you have other enchantments on the weapon? There is a character limit to the name of the weapon, and every time you enchant it adds to the description, eventually resulting in a crash if you don't shorten the name somehow. I usually totally wipe out the game's title and name it something I can appreciate better, i.e: Longsword +1 +2 +3 Lawful Keen Icy Burst Axiomatic becomes The Holy Avenger.
