Stocks opened higher Tuesday and appeared headed for solid gains, peaking at 689.17 about 30 minutes into the afternoon session. Had the index closed there, it would have been a fresh eight and a half year high. However, the market slid 1.3% over the next hour to 680.24 before a last minute spurt higher allowed the index to eke out a small gain of 1.15 points, or 0.2%, to 684.20.