... Memtest shows no RAM problems.
I noticed on my 3000 that if I put the 2GB stick of RAM in Slot 0 and the 1GB stick in Slot 1, it doesn't boot.
It does boot with the 1GB stick in Slot 0 and the 2GB stick in Slot 1...
I would try different RAM sticks from a different manufacturer
. The reason being that certain systems can be very particular with certain manufacturers' RAM. I don't know why but Hynix brand seems to work perfectly in my T60/R60 systems but some Samsung (IIRC) sticks gave me a hassle. I've learned through pure experimentation that a T60 and R60 I had would not
boot with 1GB and I think also a 2GB stick I had, which were good and worked perfectly on another laptop requiring DDR2 memory. So Memtest may not show any problems, and the stick of memory may work perfectly on another system or in one or the other slot (that is, in a certain configuration), but there seems to be something in the nature of the construction of certain RAM sticks from certain manufacturers interacting with certain memory configuration requirements on certain systems
which seems to cause problems either not booting up to begin with, or booting just fine but and then causing random freezing on certain systems but not others ??
This is just an educated guess on my part.
Good luck and have a great day