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First I'd suggest moving all these add ons to the desktop temporarily and lowering your rendering settings. In the past when we've heard about this issue, it's been due to scenery loading getting too far behind. You can test if this is the same problem by pausing the simulator and waiting to see if the scenery eventually shows up. This could take a very long time--we've heard about it taking 25 minutes or more. If you try these tests, I'd recommend filing a bug report here so we have the results info.
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