At 141K miles, there may be enough wear on the rings and valves to cause that slight deterioration in performance. Wasn't 0-60 time when new right around 8.0 sec.? I'm assuming that your plugs, wires and fuel system are all in good shape.
You might want to perform a compression and leakdown test to get an idea of the condition of valves/rings etc.. Even though the engine is pretty robust for high mileage as long as the oil is changed regularly (except for the valve guide seal problems that affected the 86-87 model years), there is likely to have been some wear in those parts. A "tune-up" won't correct for that.