Have seen this several times where a transmitter went spurious. The worst have been Johnsons. It's often temperature related, at least the frequency of the spur. A spectrum analyzer and a directional antenna are your best tools. A portable scanner is also very handy.
In one case we tracked down it was a small two watt telemetry transmitter, not a pager, trashing a UHF repeater ten miles away when the temperature was between 40 and 50 degrees. We found their spur was there any time their transmitter was up but only on the repeater input at certain temperatures. This particular transmitter was owned by a pipeline outfit. When we finally were able to contact them their tech was quite knowledgeable and replaced the offending transmitter. And they thanked us for bringing the problem to their attention. With other entities sometimes we have not been so fortunate. Often when an offending transmitter was identified we got denials of responsibility from the owners but persistence and pressure from authorities have generally paid off. Good luck, Al, K9SI
