The lowest levels of agreement were in November 2001 and July 2002. I assume that those are part of the general tendency to rally around American institutions that occurred after Sept 11, 2001. Apart from that, there's not a lot of change.
However, closer examination suggests a tendency for agreement to decline: in a regression including a variable for the wake of September 11 and a time trend, the coefficient on the time trend has a t-ratio of -2.78 (controlling for autocorrelation, it's -2.25 or -2.36). This really surprised me for two reasons. First, I thought that conflict involving the President and Congress would lead to more negative views of "government" in general. Second, although anti-government sentiments were strong in the Republican party in the late 1980s, they seem to have become much stronger since then. As a rough index, Grover Norquist was mentioned only twice in the New York Times through the end of 1994, and over 1,000 times since then. As a result, I expected views of "government" to become more negative at least among Republicans. Maybe answers to this questions are affected more by routine dealings with government agencies, which probably have become more positive over the period (see my post on the Postal Service).