Help, before I shoot my machine!

2 yr old IBM NetVista 2254G23
1 GHz Intel P3
512 Mb RAM

I have a problem that I've been trying to fix for 3 weeks. Here's a quick
list of some of the symptoms:

#1. Certain web pages containing Flash elements freeze the computer. Not
ALL pages with Flash locks it up, just certain ones like
https://webauth.comcast.betb (my broadband provider). The Macromedia site
works fine, as do other Flash sites.

#2. Opening certain images in Photoshop7 or trying to edit / manipulate
others freezes the computer. Oddly enough, the same images or operations
have no problems in Illustrator 10 or ImageReady.

#3. Receiving email from my default address in Outlook Express doesn't
freeze my computer, but it doesn't complete and I have to abort the
transfer. I can receive it all perfectly well in Outlook, but OE gets stuck
right in the middle of receiving the transfer.

Other than these issues the machine works fine.

After updating every driver / program / peripheral / BIOS / etc I finally
narrowed it down to the memory. I have two 256 Mb stick in each DIMM. If I
take out one of the sticks, everything works fine... the web pages,
Photoshop, Outlook Express, etc. It doesn't matter which stick is in there,
they both work perfectly alone. I even purchased a new stick just to be
sure. All 3 work perfectly by themselves, but no matter which 2 I use or
which DIMM I put them in, none will work with any other.

It's not the DIMMS, it's not the sticks. So what is it about putting in 512
Mb of RAM that could cause these problems?

Thanks in advance.

-Skip