You can also check in the Tools | Options | General section if you have the option "No partial loading of the documents" checked or not. If it is not checked, TopSolid will only load the visual reprentations (lite weight). If it is checked, it loads alot more.
Also, are you running in 64bit or 32bit? If you are working on very large assemblies, I would recommend moving to 64bit and adding more memory. It could be the solution...
Missler Software Inc