ran007 with the engine off get under the tractor and have a look/see at the linkage while someone slowly moves the forw/rev pedal. Maybe a solution there since you did adjust it, providing of course you looked at all the obvious things first...like is there a huge mouse nest next to the the air filter, does the choke open/close properly, old gas? etc. etc.
edit to add I'm don't mean to suggest you are to blame in any way, shape of form...that's what I would do if it were me in your shoes.
geesh... my response still doesn't sound right sorry about any misunderstanding and good luck find the cause of this problem.