Sounds like an overheating issue to me as well. Perhaps the fan is not functioning. I essentially do split grade filtering with my dichro enlarger.
- First I make a test strip at 2 second intervals with the magenta filter @ max to find my time for the blacks. The Cyan stays at 0 and is not used in B&W printing.
- Second I make another test strip starting with the magenta filter @ max using the time derived from the first test strip, leave the test strip on the base/easel, then reduce the magenta to 0 and set the yellow to max and expose at 2 secon intervals to find the time to fill in the highlights.
- Third I make a print by setting the magenta back to max and yellow to 0 ant expose with the time derived from the magenta test strip, then reduce the magenta to 0 and set the yellow to max and expose for the time derived from the yellow test strip.