I have a mini water-pump/motor hooked up to an LM338 (similar to LM317, but capable of higher current output).
I set up the LM338 to create a 6.1-V output, from an 18-V input; this was done by setting R1 = 120-Ohm, and R2 = 470-Ohm in this circuit:
CASE 1: Using a DC bench Power-supply unit == SUCCESSFUL
When I power the regulator input with a power-supply unit (rated up to 3-A at 18-V), the motor runs off the regulator output very well.
CASE 2: Using a Laptop power adapter == UNSUCCESSFUL
But when I instead power the regulator input with a Universal laptop power adapter (rated for up to 6.5-A at 18-V), the motor exhibits this behavior: it ramps up and then within 1-2 seconds, slows down, and then after 3-4 seconds, runs much slower or not at all.
I also noticed the following in CASE 2:
when I initially touch the motor's V+ cable to the regulator output, it's creating a little spark at the contact.
the regulator gets really, really hot to the touch.