hi guys, imma noob in cars but what causes jumpy engines? like RPM drops to 5 then 10 then 5 then 10 again?

how is this related to Merz? it is happening on my W126 300SE. just now only car died otw to train station, lucky reached the station at a place I can park

conditions I see when it died and cannot start anymore

1) temp close to 80`c
2) no petrol going into engine

#2 is said by colleague who came to the rescue.

another probability, could it be the spark plugs?