I used to have the same problem for ages, doing exactly the same as you by sucking water thru, not great. I finally solved it. The feeder hose from the tank to the pump needs to have an inverse U bend. I mean it needs to rise up about 20cm before falling back down to the pump. This ensures that no airlocks ever get into the pump as there will always be some water dropping into it. I did this about 3 months ago and have not had a problem since, I cant believe it was such a simple solution.
Hope that works for you. If I haven't explained it well enough let me know and I'll post a photo