Motor will only run properly with a meter inline, will only pulse without
A &quot;550 motor&quot; or originally Mabuchi RS-550 is a size/style of brushed &quot;can&quot; motor most widely known for its use in a traditional radio control car, or possibly re-purposed for an airplane.  Similar motors are found in some portable drills, other devices, etc.
These are fairly powerful motors, but operate on low voltage.  Windings vary by application but are always high current.  Those intended for an RC car are wound to draw extreme current when spinning, which means they'll draw even absurdly more when just starting up, or worse, stalled.  Most affordable power supplies cannot handle such a motor; these motors are meant for high power density rechargeable batteries.
Basically what is happening is a repeated cycle where the motor gets some power, but trips the over-current protection on your under-capable power supply.  Then after a bit the power supply protection circuit resets, the motor gets some more power, and trips the overload again.  And again and again.
Really your motors and your power supplies are fundamentally incompatible.   The best path would be to buy more suitable motors, or else rewind the motors with more turns of finer wire, making them a higher voltage, lower current design (this actually was done - but typically by hobbyists going the other way.  Of course today, no self respecting RC hobbyist bothers with a brushed anything...)
You could try something extremely kludgy like putting a very high wattage power resistor of a fraction of an ohm in series with the motor (essentially what your meter is...), but that's wasteful.
Possibly you could use a very high rate PWM speed control, and limit the duty cycle to reduce the current sufficiently.
But really at the end of the day, you have parts that just aren't meant to be used together.