My mother's garage door died recently and it turned out the plastic gears inside were stripped. She contacted the manufacturer, got replacements, and I switched them out. It wasn't difficult at all and only took about an hour.
This may not be your problem, but before you totally replace, you might want to check for this by peeking inside the gear box.
Sounds like the drive gears are stripped(it's a chain drive), and I'll open it up to check it out this weekend, but the unit is at least 8 years old and was the bottom of the barrel when it was new.
I suppose I should count my blessings that it's lasted this long.....