Sewage in sump pit

I need help. We have been in this house 3 years located in suburban Chicago. We are on village sewer. Within the last 2 months we have had sewage back-up through our downstairs bathroom after heavy rains. The weird part, is that it also comes into the sump-pit. The sump pump generally works as it should, no problems, but the last few times it gets clogged with paper, cigarettes, etc. (its sewage), so I pull it out quickly let the back-up work while I clean the filter and re-drop it in the well. We have called the village and they have cleared a blockage, which as soon as they did that, it stopped coming in the sump and toilet. A few weeks later, the same thing occurred, including the call to the village and clearing a blockage (although in a different location). They came out to do an inspection, and found our sump pump was up to code and done the way it should be, but they couldn't figure out why we were getting sewage. Any thoughts? There are three pipes leading into my sump pit, I am assuming 1 or 2 are drain tiles and one is an odd sewer connection. No water goes into the pump when we use the toilet or sinks. What can I do to prevent this? We have a backflow valve that needs servicing (the handle just spins), but am afraid if I close the sewer valve, the toilet will stop backing up, but the sewage in my sump pump will be worse. One last thing, unless it rains the sump pit is bone dry, and it takes a good 2-3 days of rain before water is high enough to make the sump pump go on. Sorry for the long story, just trying to give as much info as possible. Any thoughts or help would be GREATLY appreciated.

if it accessable, put a backwater valve on the pipe that the sewage comes into the pit through...
They are check valves made specifically for sewage/sewer lines.
http://www.jrsmith.com/products/backw/bwv7000.htm

Thanks for the help Mark, I will look into that. My only concern is, where will the sewage go? Is there a bigger problem I am missing? If I close off the toilet line, and the line going into the sump, will sewage just find the next lowest point (which would be my kitchen sink)? That would be a rise of about 5-6 feet. I can handle throwing a pump in the toilet and sending out the basement window better than I can handle sewage in my kitchen sink.

Hitquick,
did you ever find a solution to your sewer in sump pump problem??? I live in the chicago burbs too and my problem is identical!