Automatic transmissions rely on a throttle position signal to govern shifting and transmission oil pressures, and although that's done electronically on some late-model transmissions, most automatics use either a vacuum diaphragm unit (called a modulator) connected to the engine by a hose, or linkage (usually a cable) to the throttle assembly. A few transmissions have both.

Even if the transmission shifts have not yet become objectionable, make the following checks as a preventive maintenance routine, because lowered oil pressure could allow some transmission slippage, which translates to premature wear. Refer to your service manual for the correct linkage adjustment.

Unplug a modulator hose connection at the transmission. If you find oil inside, the modulator diaphragm is defective. Inspect the hose itself for a crack, and replace if necessary. Push the hose back on all the way. If the hose is stiff, and prone to leaking because it no longer grips the fitting, you need a new piece of vacuum hose.

Clean all external transmission shift linkage joints with a spray penetrating solvent, allow it to dry, then work chassis lubricant into the joint. Check the hoses and clamps in the lines that run to the transmission oil cooler. Seepage form these connections can be difficult to trace, as it's often far from the transmission.