too many TBF cong happens for two reasons :
1/ your abis is totally overloaded
2/ the GP board (or GPU) is totally overloaded
Check if the cells which are congested are connected to the same GP board ?
If yes -> all cells with the issue are connected to one GPU and are suffering at the same time all together –> you should add more GP boards per BSC in your MFS.
If not -> the problem occurs only on random sites in your network -> add more extra nibbles on your abis’s.
In both case, you should actually be able to see the congestion on some other JPI’s as well : check out ABIS indicators, ATER indicators and MFS (DSP) overload indicators.