Experts say kidnapping by strangers, the crime so dreaded by so many parents, has always been rare and is probably on the downswing.  From San Diego, Salt Lake City and Stanton, California, the grim news reports suggest an epidemic of child abductions.  FBI spokeswoman Angela bell said yesterday, Any child that's missing is one too many, but the media publicity that these cases are getting making it seem like a big jump, and that's just not the case.  Most abductions are carried out by relatives of the child as part of a family dispute.  Statistics indicate abductions of children by strangers are declining.  Bell said the FBI opened investigations in 93 such cases last year, compared with 134 in 1999.  Some abductions by stranger do not result in FBI involvement, but the National Center for Missing and exploited Children estimates the total number of cases annually at 100 now, down from 200 to 300 in the 1980s.