My '91 420 SEL (199,500 miles) idles and drives smoothly but has recently developed a hesitation or intermitent stumbling when accelerating hard. It is most noticable at higher rpms. My mechanic checked the injectors (replaced 1) and could not find any vacuum leaks. I installed new plugs. The old ones showed even combustion. The fuel pumps, gas filters, distributor cap and rotor were all replaced over the last year. I did a check on the resitance of the spark plug wires and found 2 reading 1250 ohms, 5 reading 1300 ohms, and 1 reading 1400 ohms which is out of spec. I think the wires are the problem. Any other ideas?

J.Chip