Slowing sales and traffic brought the National Restaurant Association’s restaurant performance index down for a fourth straight month in September.

But it remained above 100 for the seventh consecutive month, a benchmark that indicates expansion.

The index stood at 100.2 in September, down 0.3 percent from August.

Uncertainty over events in Washington, D.C., also contributed to the drop.

“Restaurant operators’ confidence in the economy continued to deteriorate, which was likely due to the fact that the government shutdown and debt ceiling debates occurred during the midst of the survey’s October fielding period,” Hudson Riehle, senior vice president of the NRA’s research and knowledge group, said in a news release.

Riehle discusses the index and other economic indicators in an online video.

Restaurant operators reporting a same-store sales gain between September 2012 and 2013 totaled 41%, down from the 53% reporting higher sales in August. And 40% reported a decline in same-store sales in September, up from 33% in August.

Traffic levels also dipped in September, with 33% of restaurant operators reporting higher customer traffic year-over-year, down from the 45% reporting gains in August. And 44% of operators said traffic was down in September, up from 38% in August.

Their outlook remains comparatively cautious, the trade group found, with 34% expecting higher year-over-year sales in six months, down from 36% in August and the lowest level in 11 months. But 13% expect sales volume in six months to be lower, down from 16%.

Responses to the nationwide monthly tracking survey showed operators are less optimistic about the economy’s direction. Only 19% expect conditions to improve in six months, down from 23%. And 28% expect conditions to worsen, up from 22% and the highest level in nine months.

But a majority plan capital spending. According to the index, 52% plan to invest in equipment, expansion or remodeling in the next six months, up from 45% in August.