I'm thinking of buying a 1990 model 90 quattro 20v. At $1,200 it seems like a great buy except that it cuts out (loses power, requiring a down shift) when heading uphill. Anyone delt with this kind of issue? Are these models worth putting money into?

I looked into buying a 20V myself. Unless it is in pretty good running condition I wouldn't touch it. The 20V motor was not very common so getting parts might be a "stealer only"! Check out 20V.org and you will probably get a better answer.