First thing, you need to establish the following:
1. What is your spectrum range?
2. What are your near spectrum neighbors?
3. There are different kinds of spectrum usage in your country?, i.e., your country use rules of America and Europe at the same time or only one?
4. There is an inventory of spectrum usage in your country?
5. Did you make a spectrum sweep before you put in service your system?
Why I asking you about that? Well because in our country some things like I mentioned before are happening, we have 800 Mhz cellular system and 900 Mhz too, this is bad because one operators is using P-GSM and it overlaps by 4 Mhz the 800 Mhz, then P-GSM was suffering and they think that we were having intermodulation or harmonic components, but the real thing was that because the filters of BTS are standard they receive our downlink power into its uplink, in other words, the filter of P-GSM for uplink is from 890 Mhz up to 915 Mhz then they receive the full power of any BCCH in the range of 890 Mhz to 894 Mhz. The case is worst if you use E-GSM this is from 880 Mhz to 915 Mhz and R-GSM is from 876 Mhz to 915 Mhz.
By the way in America the 900 Mhz is an ISM band then we have a lot of devices like cordless phones, toys, automatic gates, etc.
If you don’t have spectrum conflicts and the less likely situation about interference from ISM, because your country would need to have many devices of that kind and working at the same time or almost all the time, then the most likely is you are facing a JAMMER with high power located in the worst location, for example a hill in the middle of a valley, or less probably but not impossible maybe a very powerful broadcast transmitter, like TV, FM, we face that problem in the past with a broadcast from TV that have a broken filter.
You need to take hands on spectrum analyzer and see you worst locations at the time you are have the worst situation in order to have a great opportunity of catch the interference and please don’t see only you spectrum see below and above because you will get more chances of catch it.
I hope this can help you, best regards,