After add memory make the imac very slow, anyone can help?

Dear All,

I have a iMac 27" (Late 2010 model) with 4gb 3.2GHz i3 running on latest 10.6.7. It is running prefect and i love it, however, it is suffer from short of memory as sometime i run parallel desktop therefore decide to 4g more.

I have look for many info and as many people suggest, added 4g Samsung SO-Dimm DDR3 1333, the nightmare start from there.

Bank 0 Dimm 0 2g (Original)

Bank 1 Dimm 0 2g (Original)

Bank 0 Dimm 1 4g (New)

Bank 1 Dimm 1 None

It is fine to install the ram, and everything look fine at first, Mac can locate the ram and even it fail/slow to response at first boot, after reboot it look fine.

However, i find it is sometime running really really slow and even not response for a few second to even a minute, the HDD seem keep running for every few second, sound like it is reading/writing something.... and this kind of sudden hang keep happen every minute or two....

I start to search for the problem, and make a disk utility check, verify the HDD and it say everything is fine for the HDD. Then i have run a boot up hardware check and it report everything is fine including both HDD and ram.

After that, i use Onyx to try to remove all sort of cache, it look better for a while but the situation happen again after awhile.....

I am really headache after the situation happen for few day and after i have try every method i can find, is there anyone have idea what's wrong with my Mac? Please help!!!!