I had a similar problem, but for me it was CS 1.6 and CS:S.
I found out that it was actually due to this update that Steam issued, but I think it was fixed in the latest update. It affect computers with Dual Core Processors.
I had a great ping, and normally, my fps was 60. But after the June update, it dropped to 5-15. After an update last week, it’s back to 60 again.