hard start on the 92 w140,300se
 

My new (for me) 300se,w140,needs a long key to start,regardless
if the engine is cold or warm,it might be the gas quality?,it just
starts slowly the rpm gage goes up slowly when it starts,any
other car will start at high rpm and then go down to regular idle
after 1-2 seconds.When engine cold,it starts at 1100rpm,stays
there for 10 sec. then drops to around 700rpm.,sometimes if
I do not let it warm up for 1 minute or so,if I turn the wheels
sharply will die...I have to restart it.If I step on gas when cold
while ideling it might even backfire shortly,Iallways have to let him
warm up a little...any suggestions? Thanks.