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It does sound like fogging. What sort of camera is this? First make sure that you eliminate mirror fogging as a cause. Primary mirrors can fog up as well as secondaries. Small blower fans can act as demisters for the primary/secondary if blowing across the face of the mirror, but don't use big fans as they can encourage tube currents. You may also have the front low pass filter (if a DSLR) fogging up. You could try wrapping a dew heater strap around the focuser to try and heat the camera adaptor to get some to transfer to the front low pass filter. I run a filter over the front of my DSLR coma corrector and adaptor and fill the space behind it with argon - this prevents condensation/fogging. There are front low pass filter heater options that get discussed in the Cold Finger thread but you need to disassemble the camera to get to it and have to know what your doing with electronics to install a nichrome wire heater around the filter.
Of course there is always the option of staying up and monitoring the capture and stopping the run if fogging starts.
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