This has been a topic quite a few times. You say you lubed it, but unless you get up under the tractor a bit, following the linkage and use something like PB Blaster or WD 40, you don't get the lube in the right place.
If you follow the linkage, moving the pedal while watching, you'll see there is a rounded piece on the side of the transaxle that moves back and forth. Spray up in there with the PB Blaster (preferable to WD) move the pedal back and forth a few times and spray again.
If you have trouble locating the spot, let me know and I'll post a picture of the area.
This should take care of the problem, and I do it at least every week or so.
John