No ccecilm, you are not the only poor sucker who bought this machine.
I bought mine (SST16) new in 2001 for $4500.00. I am mowing 3 acres with it & haven't had much in the way of trouble until this year, aside from the hill mowing problem.
Mine wont steer very well at all. The local shop wasn't much help ."..maybe your engine is bad." was the response after a phone call to them! I perform the same annual maintenance you do. I replaced the vacuum filters with very little results. The John Deere site of course says to consult my local dealer for the tech manual or call the 1800 #.
Is there an online source for the tech manual, maybe PDF format?
I believe my problem lies with the steering actuators/switches or in the vacuum lines.