MDNSResponder keeps taking up more memory. After restart, it takes up about 5MBs of memory; after a few days, it is currently at 165MBs, and that's not the highest its gone. Currently running 10.5.8 with all updates.

Question

I'm running an Intel Core Duo iMac with 10.5.8 and all updates.
The computer has been running slow, even after replacing the HDD a few months ago. I noticed last week in Activity Monitor that the process mDNSResponder was consuming several hundred MBs of memory. I searched, thinking it was malware, but found that it is the correct spelling of the process and is vital for Mac networking. I restarted the computer, and mDNSResponder shrunk to only consuming about 4MBs of memory.
I checked again a week later, and it's climbed back up to consuming 165MBs of memory, and seems to be climbing.
Is this normal? Between it and Safari, there's always at least 300MBs of memory being consumed, just being connected and using the internet.
I've read elsewhere that sometimes Brother printer drivers can cause mDNSResponder to consume much of the CPU; however, I don't have a Brother printer, and mDNSResponder isn't using any of the CPU currently. I couldn't find anything posted about a possible memory leak.
When I open the process in Activity Monitor, it says there are zero hangs.

Answer 1

The amount of memory it's using is largely irrelevant - at least given the available data.
One of the things mDNSresponder does is cache DNS records, so of course it's going to use less memory after a restart (it has no cached data) than it is after a couple of weeks of runtime (it's cached a slew of lookups). In fact, generally you want this process to use memory since it improves overall performance.
The exception to that rule, of course, is if you don't have enough memory in your machine to do the other tasks, in which case that 165MB might be better used by other processes to reduce paging, but that isn't apparent from your post since you don't include any kind of memory stats. If you have plenty of memory available then the 165MB used by mDNSResponder isn't an issue.
Look more closely at your memory utilization and processes that have high CPU usage rather that ones with large memory footprints per se.