Adjustable Mini Pumps?

Hi:
I bought my wife an outdoor wall fountain and the pump that came with it
is intermittent and unpredictable. Sometimes it stops working, other
times it's too strong & the stream splashes all over. I suspect that at
least part of the problem is that pieces of leaves etc can get caught in
it, since there's no filter. Can anyone recommend a good replacement
pump for this application? I'd really like one with a good range of
adjustment so I can turn it down to just a trickle. If most of these
mini pumps share a similar design (ie with no filter) has anyone had any
luck wrapping a filter material around the pump to keep it from jamming?
Thanks in advance,
Freud