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Hi jps - thermal equilibrium and seeing as DP mentions may well both be contributing to your problem. However, both factors are more "take what you get factors" whereas for the third factor mentioned, collimation, you can actually do something about it and it's something you can easily rule in or out as an issue with a simple star test (you can get plenty of fancy collimation tools, but the best collimation tool is a defocussed star image). After an hour or with the scope outside start with an eyepiece that gives you say 200 times aim at a ~Mag 2-3 star and then defocus slowly. You should see the star dissolve into a series of concentric rings of varying brightness around a central point (the central rings themselves may be very faint from the effect of the secondary mirror obstruction). If the rings are clear and perfectly concentric at this power you're collimation probably isn't "way out" (to test and adjust critical collimation you'd need to up the power further) - if it isn't nicely concentric then your collimation is pretty off and this will really upset your views. The links DP provided should explain how to do the actual collimation if you find it lacking.
cheers,
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