Incorrect "symbol not resolved" errors

I have a large ASP.NET MVC (not Core) project, which builds and runs without error.

Every other day, R# decides that two particular extension methods from a referenced assembly cannot be resolved. Everything else from that assembly is fine. Hitting "F12" on the methods in question takes me to the definition of the method in the object browser.

When I clear the R# caches and restart Visual Studio without making any other changes, R# can magically see the methods again.

I would normally chalk this up to the R# caches getting corrupted. But it's the same two methods every single time.

Is there anything I can do to narrow down where the problem is?

VS2017 - currently 15.5.1, but it's been happening for several versions now;
R# 2017.2.2
Windows 10 Pro 64-bit 1709