Sewer smell in basement -- not all the time
I just bought a house in July 2008 and it was built in 1974 - I have a septic tank. Occasionally the basement smells like sewer gas, but not all the time. I noticed that the basement smells like sewer gas at different times, sometimes for no reason, and smelled when the downstairs shower is used and when it rains (sometimes). The two drains in the basement floor also do not drain - the water does not come to the top, but I can see it sitting in it. I tried to put Drano in them and then flushed it out, that's how I noticed that. Another thing I've noticed is that the toilets function just fine, but when I was cleaning and dumped water from a bucket into it, the toilet did not go down as it is supposed to.
Do you think Drano and toilet Drano will do the job if I put it in all the drains and the toilets? And keep doing Rid-X treatments? Or does this sound like something serious I should call Roto Rooter or another professional to do the job? Please tell me I can do this myself - I do not have the money right now as I just bought the house. :)
Thank you for your help.