problem with Hp monitor and graphics card

hi , i have a hewlett packard t550uk desktop with a hpf1703 tft monitor.
all was working well up until last month when the computer started acting funny. sometimes it would just freeze up and sometimes it would freeze with multicoloured spots and dashes on the screen which could only be remedied by manually turning it off. Then when i turn the system on and then the monitor the screen is black and it reads " warning pc entering power saving mode" the button on the monitor then goes from blue to orange which means that it has entered power save mode yet the desktop is working fine as it still makes startup noises.
i have tried using a different graphics card, i have taken out the 256mb asus n vida one and replaced it with a 128mb ati radeon celestica card, after installing the card it kind of worked but it would not let me watch any kind of video it said i didnt have the necessary hardware.
when i turned the desktop off and back on i was confronted with the same problem, the monitor kept going into power saving mode and turned off automatically.
if any one knows of a solution to my problem i would really appreciate it, i have been to the shop where i bought it and ive been given a new monitor but the problem persists so i beleive the problem is elsewhere

many thanks for your help in advance

Did you change the video drivers when you switched video card? If not that will cause alot of problems. Also make sure the processor fan is working and not clogged with dust.