To throw a little advice your way Sandman, it sound's like your pump doesn't have ENOUGH mustard to get the job done, I would suggest considering exactly:
1)How much H20 your reservoir is holding
2)How much your system is holding when its flooded(how much less is in your reservoir)
3)how far your reservoir is from your system
I.E.-from the very bottom of your reservoir to that three point nozzle
and your should be able to roughly gauge how powerful of a pump you need, matter of fact you can call any good supply shop you find online and specifically ask them which pump to get based on those numbers.
just a few:
-http://www.ecogrow.com/
-http://www.bghydro.com/BGH/items.asp?Cc=WP&iTpStatus=0&Tp=&Bc=
Even,
-http://www.amazon.com/EcoPlus-Submersible-Water-Hydroponic-Pump/dp/B0013KGO1A
As far as pumps go, a pump is a pump. buy it from the pet store or some crazy hydro store, If the pump is rated at a certain GPH doesn't matter where it came from.
hope this helps....
-moose