I had one that stumped me like this one time and it ended up being bad eq's that we reused from the mod that we took out. Also check center conductor on input sometimes signal looks ok on input but once it goes thru the amplification process (nonlinear) reflections will manifest themselves. This can also happen going thru splitters even tho that is linear I believe.
This is CABL.com posting #330950. Tiny Link: cabl.co/mbyf4