MISCELLANEOUS; Some random thoughts

* It's hard to imagine any superstar with a greater disappearing act than speed skater Eric Heiden, who emerged recently to collect the Sillivan Trophy as the nation's outstanding amateur athlete. Since his five-gold medal sweep at the Lake Placid Olympics, he has given up skating for cycling and enrolled at the University of California, San Diego, where he's studying biology.

* The best current pro rivalry may be that between basketball's Boston Celtics and Philadelphia 76ers. Both clubs are winning at an .800 clip, a pace rare ly achieved in the NBA, and play in the same division.