[Hampshire] trouble after memory upgrade

Hello,

this is an Acer Aspire T180 desktop:
Linux ss07 2.6.24-19-generic #1 SMP Wed Aug 20 17:53:40 UTC 2008
x86_64 GNU/Linux
Came with 2*512MB ROM, with 4 slots.
Bought in November 2006.

I have just got 2*1GB sticks from Crucial, and moved the current 1GB
from banks 1,2 to banks 3,4 (it was recommended in the little note
that came with the memory that smaller-capacity units should occupy
the higher slots), putting the new 2GB in slots 1 and 2.

At boot up attempts we always get to the GRUB prompt, then:

A.
1. Select the current kernel, gets as far as 'Starting up...' then freezes.
2. Select the current kernel, see some panic trace for a moment then reboots.
3. Select the current kernel, see 'Kernel alive and the next line
then reboots.
4. Select the current kernel, see 'Loading, please wait...' then freezes.
5. as 4, but with a scrambled screen of 4 'loading' messages dotted around it.

B.
On to memtest86.
1. crash after 22sec on test2 with a Stack Fault.
2. reboots after 15 sec.
3. stack fault after 3 sec.
4. reboot after 15 sec.
5. stack fault after 53 sec.

C.
Remove the 2*512 MB sticks leaving just the 2*1GB new memory.
1. Boots into Ubuntu 8.04 fine.

D. Put the 2*512MB back in,
1,2. 2 memtest hangs
3,4. 2 memtest reboots.
5. Recovery mode Ubuntu immediately reboots.

E. Swap round the two 512MB sticks, having brushed the slots and
cleaned the memory contacts with an eraser.
1.Ubuntu start hangs at 'Kernel remapping...'
2. memtest runs ok for 2 minutes (!)
3. memtest reboots after 3 sec
4. memtest stackfaults after 1'45''.

The old memory was behaving OK (at least, not breaking) before I opened the box.

I am now running with just the 2GB new memory, and apparently things
are behaving.

I was wondering, since
a) the high memory banks 3 and 4 were previously unused before I moved
the old memory into them,
b) the machine is almost a couple of years old,
c) it is in a not-optimal environment (had a bit of dust inside)
d) the misbehaviour seems a bit erratic during and between trial runs above,

this makes me suspect that I have not-good contact between the 2 512MB
sticks and the slots on the motherboard.

Anyone got any comments/suggestions?