My 89 56sel with 164,000 miles is not downshifting properly under full throttle acceleration. Under normal driving conditions it accelerates and shifts beautifully. The problem is evident when driving in the highway gear (I am assuming 4th gear) and there is the need for maximum acceleration as in a passing situation. I have replaced the kickdown switch under the gas pedal so that does not seem to be the problem. I have read where some have suggested the O2 sensor or catalytic converter could be the problem. Others had suggested transmission fluid and filter change. I prefer to avoid trying one thing after another without knowing what is the actual problem. I am thinking someone out there knows a lot more about this problem than I do. Any help is greatly appreciated.