computer slow ubuntu 12.04

I have a Fujitsu Siemens compter Scaleo P working on Windows XP pack 3 with on board:
Intel Celeron 2800 MHz, Bus 533 MHz, Motherboard Fujitsu D 1740 ande a Intel 82845G/GL/GE/PE/GV/Graphics controler. The internal memory is 2Gb.
I have installed Ubuntu 12.04 (duel boot with windows) with alle the updates on a seperate harddisk.
When I start the computer with Ubuntu at first it works oké. Then at a moment(e.g. when I use Firefox more than one tab) the monitor gets black just for a moment, then the monitor is working again, but very slow, the whole computer is slow. Colors are first changing. Often only the mouse is working but nothing happeness.

Is my computer to old?? Ore is it something else.

I hope someone can help me.

Thanks and kindly regards

Gerlof Brinck.