Uptime shouldn't be the cause, but just shows how long whatever is causing the problem has had to increase memory usage. Under normal circumstances, memory usage shouldn't change no matter how long mIRC's running. I've left mIRC running for just short of 8 weeks without the memory usage ever hitting 100MB, let alone over a GB. And that's while running Invision, which is a pretty big script.
The most likely causes are hash tables, sockets, and perhaps window buffers. DLLs and COM can also cause this, but the pasted results show that there are 0 of both of those items in use.
In the end, the most likely cause is a script that is not properly closing *something* and is opening many of that *something* until a lot of memory is being used.