How can I fix the drive on Performance Power 420 self propelled mower?

The drive mechanism on my Performance Power 420 self propelled mower works intermittently. I have adjusted the cable up and down - it appears to make no difference. When the drive is working it engages as soon as I pull the lever. When it is not working, I can pull the lever right up to the bar, and no drive! A few minutes later with no adjustment, it works again. It is fitted with a Tecumseh 35 engine. I would like a manual for the mower too, if anyone knows where to get one. Thanks! :-)