I've had this problem for years with a variety of Windows systems, presumably IE related but experienced on Win7 as well. Basically same symptoms -- every time I select a thread (possibly a forum too) I get logged out.

In my case my work focus shifted from Windows to Linux so the problem kinda took care of itself (everything works fine on my Linux systems with Mozilla). Are you using IE or something different on Win 8.1 ? Wondering if we should try another browser on Windows to localize the problem.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8