I'll second Brian's reply. Even wear on tires does not necessarily indicate that everything is 'a-okay'. There could also be structural problems within the tires, especially the sidewall that may not show up, visually, for awhile.
This behavior is definately not normal for a W126 - my 560SEL is one of the most stable cars I've driven.
'87 560SEL 267K (177K on motor) Blue/Blue
'98 Buick LeSebre 60K (wife's car)
'56 Imperial Sedan 124K
'67 Dodge Monaco 130K (Sold)
'87 Chrysler 5th Ave 245K and going strong (sold)
'73 Plymouth Satillite 175K (sold)
'96 Chrysler LHS 80K (totaled)