Hi,
We are using repeaters for some indoor locations. We very often have the same problem when there is some problem with them. And when you change the frequency of course the interference still exists. It could even be installed by the competitor (not band selective as they are cheeper) several times it was some bad GSM gateways also. Also illegal boosters and cordless phones…
Usually we go to the site with a yagi antenna and analyzer, and try to locate the interferer. the best way is to connect to analyzer to the GSM antenna to see the level of the interference.