Output Issues on Dell Vostro Desktops

Greetings,

I recently took over as a sys admin at a charter school that has 31 Dell Vostro 230 and 220s desktops, which they were hoping to image and deploy to a new lab. 

Initially, they seemed to work fine, but after a while they've started to lose video output to their monitors. They're fairly low budget units with 250W PSUs. I've looked everywhere I could for a solution but haven't found one. Here is what I have done or know after a bit of troubleshooting:

1. At boot, fans and lights come up normally. The front power LED light remains blue throughout the boot up process. The monitor never indicates that a signal is present, however.

2. Swapping out a different monitor and/or VGA cable has no effect, nor does pulling the CMOS battery for 30 minutes before putting it back.

3. They all failed before I had a chance to image them.

4. I do not have a low profile PCIE graphic card to check the default video output in the BIOS, and I'm not sure one would work given the low PSU rating.

5. I moved two units to another room and plugged into a free power outlet, with no change in symptoms.

The strange thing is that there were, perhaps initially, one or two desktops experiencing the problem. That number quickly rose to 7, and on to the current total of 23 out of 31. I'm at a loss at this point and am crossing my fingers that someone out there may have an answer.

Thanks in advance for your help.

TJ