Hmm strange, I never had a TNT2 so I don't know how much help I can be here. But you may want to switch of all vid options and get everything to lowest possible settings then see how it runs. If it's ok try switching the options back on one at a time. Probably won't work but it's worth a shot. Im just thinking you may have something switched on that you vid card doesn't support, the TNT2's are getting on in years.
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Lady Astor: "Winston, if I were your wife I'd put poison in your coffee."
Winston Churchill: "Nancy, if I were your husband I'd drink it."