3vdc motor PWM problem

hi all. i posted awhile back about my time fountain project,
everything is up and running! it works beautifully (for a few seconds,
read on) with my new submersible dc motor pump. if you dont remember
the project, heres the deal:

I need a constant drippage of water. I do this with a 3v dc motor,
which pumps through a tiny valve then out the tube. its PWM with the
following: 555 timer, diodes separate up and down time. 1uf cap - 1KOhm
up, 1KOhm down

The timer is isolated from the circuit with a diode and a 1000uf cap,
so the motor doesn't interfere with the timer. thanks for the help
with this. The timer works perfectly, as with a piezo it gives a
steady tone.

The problem is that the motor will rev up to the correct rpm, which
yields the perfect flow of water. Then, after a few seconds, its RPMs
will drop lower and keep bouncing up and down (i can tell this because
there is an audible difference in the tone from the motor, and the
water flow fluctuates)

Any idea what is causing the motor to not stay at the steady rate?
Thanks a ton!

-sam

.

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