Aren't you expecting a bit too much from a treadmill you bought at Costco?
"The average lifespan of a treadmill seems to be anywhere from 7-12 years. Proper care and regular maintenance is crucial to a long and happy treadmill life. "
If it was working just fine the first couple of years, and / or you have not hired someone (or yourself) to do routine maintenance, I would suggest that it is at the end of life (or needs maintenance).
Try fixing it and selling it at a steep discount on Craigslist if you no longer want it.