You can try to download the Limit Adjuster and increase the streaming memory http://www.gtamodding.com/index.php?title=SA_Limit_Adjuster The engine has limitations but I don't think even increasing that would solve all your problems.
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The problem was easier solved two ways, first, by getting rid of the stupidly high poly model, there's no reason to use a 3DS Max render quality model with 200,000+ polys for a Broadway car mod, and second, by modifying the stream.ini file to let the game use more system ram. Rockstar originally had the memory and devkit memory set to 13500 (which I guess is only 13.5MB) but now I have it set at 999999.
Letting it use more ram solved most of it, but every time the Broadway car model spawned into the game, there was a VERY noticeable hitch in the framerate, and whenever there was more than one of those cars on the road, the game started to glitch up again, and even crashed at least once.
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