Prices on computer memory chips nose-dived recently, cutting sharply into group profits for some of Japan's leading makers of electronic products. Toshiba Corp., Fujitsu Ltd. and Mitsubishi Electric Corp. posted lackluster earnings but predicted better results for the coming year, anticipating growth in network computing and sales of PCs.Mitsubishi was hit particularly hard last year when prices for 16-Mb dynamic random access memory chips plunged 80 percent, just as the company was boosting capital investment in a German unit that produces the chips. As a result, Mitsubishi said it would be difficult to post a profit in fiscal 1997/98. Fujitsu and Toshiba predicted their chip divisions would return to profitability in the current business year.