Chris,
Interleaving means putting like RAM chips in slots that correspond to each other. For instance you would put a 128 chip in A1 and a 128 chip in B1, a 64 chip in A2, a 64 chip in B2, etc.
The best chance of proper interleaving is if the two chips are exactly the same, but I have used chips from different manufacturers as long as they were the same size. Presently I have all 128 chips and since most are from the same manufacturer they are automatically interleaved in all the slots. I did have one chip go bad and removed it but I still have the 11 others= 1.37G.
If you can successfully interleave you can get up to a 15 percent increase in speed according to some old data.
As to the errors, they could be related to the RAM but software troubleshooting is a first attempt.
Here's a page which explains many typical errors in 9.
Type 1, 2 errorsThis page will give you an idea of how to troubleshoot the browser errors.
Web browser errorsAnd this will cover Type 11
Type 11 errorAll those are very common in 9 and can often be remedied by dumping some prefs, increasing memory allotment or a simple reinstall of the OS. As a last resort, try putting in just one RAM chip at a time to see if you get the problem. You're right, that is the teadious way, so try the other possibilities first. I spent nearly two days testing chips before I found out which RAM chip of the 12 was bad. UGh.
At least memory is much cheaper than it used to be. Remember spending a couple hundred dollars for 64 chips? Yikes.
Have fun.