I am also running Ubuntu-9.04 (Ultimate-edition-2.2)
when I first installed it today it ran ok
but earlier when I ran it it started to act crazy and slow, like your saying.
I also have an Nvidia card too.
I was at first using this kernel
Ubuntu 9.04, kernel 2.6.28-13-generic
To fix mine, luckily Ultimate edition offers 2 kernel choices at boot menu
I chose the earlier kernel;
Ubuntu 9.04, kernel 2.6.28-11-generic
I'm not sure if that helps but it fixed mine