GM tube troubleshooting

At a Nuclear Power plant. Monitoring off gas radiation using GM tubes. We monitor two channels as indicated by the red and blue traces in the picture. channel 'A' slowly and then suddenly spiked upscale for a couple minutes then dissipated and reset. There were no plant evolutions that would have caused this anomaly.

The GM tube uses a 600V signal for ionization and a preamp with 15/5 vdc.

Would this display be indicative of a Tube failure or rather HV powers supply/ preamp?

I have more details if needed.

assistance appreciated.