Could be many things, but...
I had a similar problem in my '04. Mine would work, but then would be stuck at a certain speed or it would be off by 10 or 20 mph. These vehicles are notorious for the speedo and other instrument cluster gauges not working. I was able to replace the Stepper Motor for the speedometer. Now it works flawlessly. I know you said money is tight and this is definetly the cheapest fix. You have to find out first if it is a cluster issue or something else. If it's a cluster issue, theres a VERY good chance the Stepper Motor is inoperable. I bought a pack of two on ebay for about $13 (
not sure if the motors are the same from 2003-2004, probably are).
One more thing. Check all of your cluster lights before you take it apart, if you're in there repairing a motor you might as well replace any bad bulbs too (just a suggestion).
This is a link from Youtube on how to complete the stepper motor replacement.
http://www.youtube.com/watch?v=gWELz904fj0&feature=fvwrel
If you know how to use a soldering iron, you can complete the repair yourself. Just be sure this is the problem before you tear your cluster apart.
Hope this helps!