I have a nice 128 meg Nvidia that has been great through a 15" flat panel and a 15" crt after the flat panel went out.
I now have a 19" flat panel ( Dell $261 delivered thought I'd add that info!), and when I'm playing Doom it's choppy now.
What settings need to be changed. I ask so I don't change a bunch of stuff and get lost in my mistakes.
My guess is the 19" flat panel is running at a higher screen resolution than the old monitor.
If this is so the only solution is a new faster video card.