My pump won't start on low speed - when I power it up, it humms, but the motor won't turn. If I go to the high speed, it starts ok, and if I turn it back to the low speed without stopping, then it keeps going fine too...

I thought, maybe something is jammed in the motor or wet end, but I can easily spin it by hand with little resistance. I poked the capacitor with ohmmeter, and it seems to be charging, and holding the charge (I found the capacitor behind the back cover ... there is also a "lump" on the side of the motor - is there anothehr capacitor there? Should I test that one too?).

Any ideas what is wrong with it? Do I need a new pump? I would buy one, but don't know if it will help
Where should I go if I was to buy a new pump? Any good links on the net?
It is a 3hp GE motor with 56Y frame (I believe), 230V.
Also, do you think I only need a new motor, or the whole assmebly?
If I was to replace the whole thing, should I stick with 56 frame, or would a 48 wourk too?

Thanks a lot for your help!

the "hump" is a second capacitor, but to properly check these capacitors you will need a capacitor checker. It does sound like you have a defected low speed start capacitor.

In the event of changing out the whole motor, you can use either frame, but the 56 usually have better bearings, last longer, and run more efficently