Toilet burps and backs up

[MW]

I'm a new home owner and have three toilets that aren't working well. The symptoms are: 1) they don't flush completely and need to be flushed several times to clear the waste from them 2) sometimes they back up for no apparent reason (nothing out of the ordinary is put in) then they will spontaneously clear themselves 3) sometimes when the clear themselves they "burp" a large air bubble or three. The house is roughly 18 years old. I'm on a septic system which was pumped last December if that makes any difference. Any ideas? Thanks in advance.

[hj]

There are any number of things which could produce those symptoms, and having the tank pumped in December does not mean it could not be the cause of the problem. The tank, and effluent pit/field could also need servicing or replacement.