Fluctuating 12v power supply is giving me problems. What combination of dc/dc converters, regulators, boost, buck, step up, step down, etc. could fix it?

A bit more context: The system is a battery powered unit that has a heavy intermittent irregular motor draw (maximum .67C) on the battery which knocks down the voltage a volt or so while operating. The batteries are supposedly rated for the draw.

One other system connected to the same battery seems touchy about the voltage coming back up after the motor shuts off and responds by shutting itself down momentarily then coming back online. Unfortunately even a temporary shutdown is unacceptable.

Providing a second pack is undesirable for charging reasons and filtering doesn't work so I thought I'd add a combination regulator that would kick in and boost when the voltage was low (motor running) and shut off when the voltage was normal. Is there a less kludgy way? I'm working with about 2.5A and everything available looks really expensive (and big). Would a better way be to set up a circuit to add a sort of pwm or even just a slow changing high A transistor to slow the voltage rise?