In the biggest stock market tumble since 1991, the Dow Jones industrial average plummeted 101.52 points to close at 5,075.21. Worries that a partial government shutdown might cause the Federal Reserve to postpone a predicted lowering of interest rates on Tuesday sent investors into a selling frenzy. Declining issues led advances by four to one on a heavy volume of 426.23 million shares as of 4 p.m. EST, despite a one-hour opening delay in trading due to computer problems that stock market officials have yet to fully explain. With the Dow at near-record highs this month, the slide represented only a two percent decline in the industrial average. But today's trading wiped out most of the gains made in December and created new worries that for the Dow, up 32 percent on the year, the 1995 rally might be faltering. Hit especially hard today were tech stocks, such as Netscape, IBM and Apple, which continued to weaken after leading the Dow's charge past 4,000 and then 5,000 this year.