The main stock indexes made positive price reversals Tuesday, the latest in the market’s run of mixed signals. The Nasdaq, down as much as 0.9% early on, closed only 0.2% lower. Weakness in biotech, solar and some technology stocks hurt the composite. The Nasdaq did close back above its 50-day moving average, which made its reversal all the more interesting. The S&P 500 climbed a fraction.