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Mike,
I'm assuming your 8" dob is well collimated.
There are a number of factors coming in to play here.
If you are viewing early evening, are you giving your primary mirror time to cool down to the outside temperature? Convection currents off a cooling mirror will give stars a hot fuzzy look. Also, early evening, heat from the ground will cause convection currents in the lower atmosphere and this make seeing worse the lower the star is to the horizon. Things should settle down more a few hours after sunset. If sunset is about 5pm, by 7pm things should be settling down.
The 8" dob will gather more light. Therefore, stars will appear brighter. Brighter stars will produce more glare which is accentuated by poor seeing and poor transparency.
Regards, Rob
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