|Not to pick at nits but, you said, "it uses resources and can cause crashes." which is not the same thing as, "Actually it was crashing every 20 mins".|
Just for future reference, should you have issues that cause you to come to a tech help site like this again, you want to post all pertinent information. I double checked just to keep my foot out of my mouth and sure enough, no mention of the unit crashing other than what I copied and pasted above.
Had we known it was crashing, we would have probably taken a different troubleshooting track (or at least, I would have) instead of the one we did.
Having said that....
I've never worked with any SMC. For the most part, I do enterprise level networking here at work. However, we do run an in-house wireless network that provides nothing but internet access and is unsecured. Call it a "guest wireless network". For that we are presently using SOHO level equipment. We do have plans for an enterprise level in-house wireless network that will be able to provide secure access to internal resources as well as external "guest" type access for non-employees. However, due to the present economic situation, that's been pushed back until we have the budget for it.
So far I've used a lot of D-Link equipment but I found it breaks down too quickly. As a result, we've settled on Linksys WRT54GL routers running tomato firmware. I like them a lot and so far I haven't had a single one break. The third party firmware gives a lot of additional features not available on the default firmware and, it's linux based.
All of the aforementioned Linksys routers are being used as Access Points with one exception. Therefore none of them have the WAN port in use. I didn't have to disable anything or fake any IP's on the WAN side in order to prevent discovery packets being sent out. Because that port isn't in used, technically, it's disabled and there are no records of discovery requests, or any other kind, going out on the WAN side.
You may want to consider getting rid of the SMC router and buying yourself something a little better.
Also, it could very well be there is a hardware malfunction in your unit causing it to crash. Discovery packets in and of themselves won't make your system crash. But if is a hardware issue, then you most definitely want to replace the unit.