RB750GL intermittent slow problem.

Our network has a link of 20MB and is shared for 5 clients.
Everything worked perfectly, but then the problems started with slowness, even to the point of not access any internet site.
So reseted the RB750GL and restored the backup configuration. everything is working again, but after two days the problem recurred.
If I connect the modem direct link goes to 20MB but if I connect on ports 1,2,3 and 4 RB750GL speed is ranging between 3MB and 4MB 1MB.
Anyone ever seen this kind of problem?

RB750GL 5.22

Thank you and sorry for English.