Toilet acting up

[minnesippi]

Sometimes - not all the time - the toilet (lav 1)will not refill all the way. Then when flushed, its like a big bubble comes up. It may or may not refill all the way. Seems like it is clogged, but not all the time.
Now, a second toilet (lav 2) is doing it as well.
Both are on the 2nd level - lav 1 has had tank guts replaced close to every year for the last 7 years and lav 2 once in the last 7 years.
They are used regularly.
Any ideas why they would be doing this? Is it in actual fixture or in pipes? If in pipes, what could be the issue.
Thanks in advance.

 

[LiQuId]

Maybe a back grade in the pipe causing the bubble... Are the toilets both the Same Brand ??

 

[Caduceus]

There is probably a clog in the pipes. If no access is available a toilet will need to be removed to clear the drain.

 

[minnesippi]

They are both Gerber toilets. I do not know how old they are. At least 15 years.
You think a clog bigger than a plunger can handle? Would need to snake it I presume? about two weeks ago, the toilet got clogged (by my 7 year old son) and it took nearly 30 minutes of plunging to get water to drain out of toilet and flush freely. But to no avail, problem still continues.
So snake the drain. Okay. Thanks.