Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

With little economic data for investors to mull over until Wednesday, and earnings season beginning to wind down, traders once again pushed the broad-based S&P 500 (SNPINDEX:^GSPC) lower as they were left to again dwell on when the Federal Reserve might begin to pull the plug on its monthly bond-buying program known as QE3.

The U.S. central banks' $85 billion in monthly bond purchases have certainly helped to bolster the housing market, corporate refinancing, and bank lending, but we also saw just how quickly mortgage application activity dried up once rates rose by more than 100 basis points -- down 53% since their peak in early May. Any ongoing speculation as to when this paring back of its bond-buying program might occur is almost certainly going to act as a gray cloud over the S&P 500's parade.

By day's end, the S&P 500 had dipped by 9.77 points (-0.59%), to finish at 1,646.06, the ninth time in the past 11 sessions that the broad index has fallen.

Taking charge of the S&P 500 gainers on this down day was discount variety retailer Dollar General (NYSE:DG), which edged 3% higher on the heels of an upgrade from JPMorgan Chase to "overweight" from "neutral." The two covering analysts cited stabilizing dollar store business trends across the country and reduced discounting as the reasoning behind the upgrade and their price target hike to $64 from $51. While I do share some of that optimism, I also can't ignore Wal-Mart's (NYSE:WMT) weak second-quarter results, which saw same-store sales fall 0.3% versus an expectation of a 0.7% gain. Since Dollar General is competing for a very similar customer as Wal-Mart, I have some concerns that Dollar General could see some bottom-line weakness as well.

Robotic surgical device maker Intuitive Surgical (NASDAQ:ISRG) added 1.8% on the session despite no company-specific news. The rebound could be based on the assumption that fears surrounding its lowered growth forecast and ongoing probe into the safety and effectiveness of its da Vinci surgical system have been overblown. Admittedly, while I do side with this view and have seen plenty of evidence to suggest that robotic surgery is one of the waves of the future, this near-term investigation could sap Intuitive's earnings potential over the next couple of quarters.

Finally, chipmaking giant Intel (NASDAQ:INTC) advanced 1.7% after receiving an upgrade to "neutral" from "underweight" by Piper Jaffray, which also boosted its price target on shares by 10% to $22. Piper appears impressed with Intel's internal cost-cutting measures and feels if it were to expedite the processing of its Atom chips that it could claim decent market share in the tablet space. It is worth nothing, though, that Piper Jaffray's price target was just $0.08 above where Intel closed on Friday, so it's not exactly breaking out the pompons, either. As for me, Intel represents what I feel is the best basic-needs stock in the tech space and still looks like a strong buy over the long term.