If you have good vacuum at the modulator, you know the modulator is holding vacuum, and you know that the modulator is good, there's not much left but a worn transmission or driveline components.

If the valve body is worn or you have a piston sticking, you can get some very abrupt shifting behavior. When is the last time the fluid and filter were changed? If it's been a while, start there. Disgusting old fluid can cause nothing but headaches and certainly does the transmission no good at all. If the fluid has been changed recently, was it replaced with the correct fluid?

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)