Yeah, kind of.
What actually happens is that in managed speed, (speed button PUSHED = magenta speed bug), the aircraft will allow altitude variations of +/- 200' in order to avoid 'hosing' the engine's thrust up and down to maintain speed. This is the so-called soft altitude hold, which saves fuel, and gives the passengers and crew a relaxed cruise.
However, when crossing the ocean, one is usually cleared to do so at a specific mach number, and one therefore selects the cleared mach number for crossing, (speed button pulled = blue speed bug). Selecting the cruise speed/mach cancels the soft alt mode, and therefore causes the engines to 'hose'up and down a lot more keep to the selected speed/mach.