alpining: Some further thoughts to follow. But, the first thing I would recommend is swapping out the factory ejector spring, for an aftermarket one. Spring kits are available through: Rusty Wood Trading. Rossi ejector springs are notoriously way over powered. The aftermarket springs, though lighter tension, do provide smoother and easier case ejection. Usually the first thing I do with a Rossi, is replace the springs. This alone, may solve your problem. Still, there may be other issues, so...
Some ramblings:
Even with the stiff factory ejector spring, the ejector collar should move back under firm finger pressure. The fact that it is hard to move....plus feels gritty, leads me to believe that it may be hanging up in its recess. You may need to disassemble the action and check the bolt end recess and ejector collar, for burrs or rough machining. Then, polish as necessary. Likewise, check the ejector spring strut. It could be bent, or otherwise misaligned. If so, the ejector collar might be jamming a bit on one side or the other of its recess. Or, the spring strut(or rod, if you like), is binding in its slot in the stud. The spring may be hanging up on it, too. These parts aren't finely finished in Rossi rifles, so often need a bit of attention. Generally a bit of polishing.
Why only Winchester brass, is a mystery. It should eject as easily as the rest. Did other brands eject okay...
The condition you describe with cases popping off the extractor indicates that there could be a problem with extractor claw engagement.. Or the spring. Ideally, the function of the extractor is to grip the rim and hold the case against the top of the chamber, under slight pressure. As it clears the chamber mouth, the case flips upward, as the ejector pops the case forward. The case then flips up and out, as the bolt moves to the rear. Generally in the same direction. Since yours are ejecting erratically, the extractor may be causing part of the problem. Uneven extractor tension may cause cases to come out crookedly... causing them to bang against either cartridge guide, on the way out. This can cause cases to eject erratically.
The extractor claw should have a good sharp edge on it. Plus, evenly curved across its width. If worn, unevenly cut or too short, this can cause problems with function. Likewise, if the spring is weak, this can cause engagement issues.
Try this test: As you slowly close the bolt on an empty test case(same as live round), you should feel some resistance, and the extractor claw should snap firmly over the case rim. If the engagement is weak, either the claw is defective in some way, or the spring needs replacement.
But, I am not fully convinced this is the sole source of the problem.
Since both ejector and extractor need to work in harmony for best results: It is also possible that a sticky ejector may be adding to ejection timing issues, too. If the ejector hangs up or is too weak, the ejector collar fails to move far enough forward, with sufficient force.....so, the case tends to fall downward as it leaves the chamber. Usually ending up popping off the extractor claw and dropping onto the elevator. As you observed, this generally happens with the last round. Often, this issue is solved simply by replacing a too-weak ejector spring with a stronger one. But not always.....so my advice at the top of the page.
This information may be useful: Cartridge feeding issues often start at the loading gate. Very often by a loose, or misadjusted loading gate. The loading gate also acts as a cartridge stop. So, if cartridges are double feeding and one gets pushed past the stop, it jams up the action. In some cases I`ve observed, the gate spring had too much curvature in it. Once straightened out a bit, it worked fine. Also, the cartridge guides on either side of the receiver can cause feeding issues. Sometimes one or the other requires a shim to make cartridges feed smoothly onto the bolt face. Your gunsmith may have already addressed these issues. Simply mentioned for information purposes.
Finally: Likely the best person to contact for troubleshooting advice on Rossi rifles, is: Steve Young (aka Nate Kiowa Jones).. Steve is an accomplished gunsmith and authority on the Rossi `92. Simply go to the StevesGunz.com website, for contact info. or Google: Nate Kiowa Jones. That should also bring you to his site.
Pretty longwinded, but hopefully of some use.
Al