By Atossa Araxia Abrahamian Sept 27 (Reuters) - Small- and mid-cap shares rose slightly on Thursday after several days of losses, as investors hoped it would re-establish an upward trend that had prevailed for most of the year. Among the day's biggest gainers was mattress maker Tempur- Pedic International, which rallied 13.6 percent to $30.42 on news that it would acquire rival Sealy Corp for about $242 million. Sealy rose 2.3 percent to $2.19 a share. Chemicals company HB Fuller dropped 9.2 percent to $30.30 after it cut its full-year revenue outlook. The S&P Mid-Cap 400 index gained 1.1 percent, while the S&P Small-Cap 600 index added 1.2 percent. "The trend is up and remains up," said Sasha Kostadinov, a portfolio manager at Shaker Investments in Cleveland. Uncertainty remains over the looming fiscal cliff - a package of tax increases and spending cuts that will take effect at the end of the year if Congress does not pass legislation deferring some of those events - and the U.S. presidential election in November. Still, "I have no reason to think the trend is going to change," said Kostadinov. "I think people have been bracing themselves for terrible. We continue to see the news be not necessarily great, but not terrible." The market's recent sharp gain was largely attributed to optimism following the announcement on Sept. 13 of the Federal Reserve's latest quantitative easing program. In the small-cap market, electronic payment company Progress Soft Corp jumped 12.2 percent to $21.51 following its announcement of better-than-expected earnings for the third quarter. RadioShack Corp shares slid 8.5 percent to $2.38 a day after the announcement that CEO James Gooch would step down as the company makes an effort to salvage its brand. The news came just days after RadioShack was removed from the S&P Midcap index because its market capitalization was too small. The mid-cap index is up 12.5 percent since the beginning of the year, and the small-cap benchmark has gained 11.2 percent for 2012 so far. Still, not everyone is so certain that the uptick will last. It "looks like a classic oversold bounce, but hardly anything that looks like a trend change," said Shawn Hackett, president of Hackett Financial Advisors in Boynton Beach, Florida. "This tends to happen when markets trending upwards are rolling over to a top."