Transmission lag--1988 300E

Have a 1988 300E. For reasons unbenknownst to me, sometimes--and only sometimes--there is a lag between putting one's foot on the gas and the transmission downshifting.
When the transmission downshifts, however, it does so smoothly. No bucking or slipping--EVER.

I have owned several 300E 3.0 litres, so I know how these transmissions normally operate.

Is there a problem with the adjustment on my kickdown cable? Is there a problem in my throttle linkage? Or is there some sort of problem within the transmission? Fluid is cherry red, and I change it every 15,000 miles.

Thanks in advance for your advice!

Ed