Starting Problems

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Sometimes when I start my 1998 740il it revs up to about 2000 rpms then dies, It does this sometimes 3 or 4 times. When it does this I then have to rev it up to about 2500 to 3000 rpms and hold it there for about 10 to 15 seconds then let off and it will stay running at about 600 rpms. It doesnt do this all the time. Sometimes I can drive it for hours and it never happen and other times I can drive it for 30 min and shut it off and start it up 10 min later and it does the same thing. it does this sometimes in the morning and sometimes it doesnt. This is driving me crazy. Please help.