86 Mercedes 560sec wont start

i have a non-abused 86 Mercedes 560sec that turns over but will not start. One day
after coming out of a store it would not start. half an hour later a friend came to help
me tow it home. before leaving i tried it  it started and i was able to drive it home. the
next day it would not restart. (6 years latter and my some has now taken this on now
so every thing described after this is his wording) brought one of are trucks up or
tractor with fully charged gel sells i can get the car to kick when using spray start. was
able to get the car to run on its own once being jumped directly to my truck with twin
battery (i did disconnect the cars battery) it ran for about 7min then it cleanly died off.
haven't been able  to get it to go since. any help is welcome because im trying to get
this to be my daily for junior and senor year in HS.

thank you for looking
-Liam