I personally absolutely hate dealing with .stp files. In my experiece they always seem to prove the most unreliable unless going from Catia V5 to SW.
My guess is that this .stp file is a fairly decent size then and SW tends to be very cumbersome handling large .stp files. I would first try and get an .iges file or maybe the] native Pro-e file ( SW can open certain native versions).
Here is some info. from the SW help file.
Importing Pro/ENGINEER Files into SolidWorks The Pro/ENGINEER translator imports Pro/ENGINEER part or assembly files as SolidWorks part or assembly documents. The attributes, features, sketches, and dimensions of the Pro/ENGINEER part are imported. If all of the features in the file are not supported, you can choose to import the file as either a solid body or a surface model. The Pro/ENGINEER translator supports import of free curves, wireframes, and surface data.
When importing an assembly, you can control how to import individual components. Sub-assemblies are supported as well.
You can import Pro/ENGINEER surface-trim and surface-extend features into SolidWorks. These features are read in from the Pro/ENGINEER file and mapped to SolidWorks.
Version Information - Versions 17 through 2001 of Pro/ENGINEER and Wildfire versions 1 and 2 are supported.