I have constant problems with my plumbing...toilet won't flush if it rains too much..any ideas?

The last user swears she did not plug it with paper.....We have an old house by Lake Erie with lousy drainage. a small septic tank that has been pumped frequently, 3 kids . The problems really started lasf year after our neighbor dug a new well, I don't know if it messed up my leach bed or what. We have a 20 foot by 30 foot yard with leach bed, the septic tank is at least 30 yrs old-I think...Hubby dumps draino down the toilet and learned how to pump septic into a tank .he snakes the pipes occ too. But how do I get to the source of the problem??? The sinks and tub drain more reliably than the toilet- but will go "blub, blub,blub...." when the toilet won't flush... air pocket? This is so frustrating having to go to McDonalds too use their bathroom!!!! HELP !!!!