I'm just guessing, but since your problem starts after several days, the 50000 could be your problem, and it's taking a few days for the chat lines to build up. That's 50k lines not bytes, so each channel could have huge use of memory.
If the window buffer is the culprit, you can possibly induce the glitch to happen sooner by using /loadbuf to load 50000 lines from the logfile into several channels. The 50000 also governs the default max lines for your @windows, so some of them could also be tying up memory.
