Not sure if this applies to CS:GO the same way it does with CS 1.6, but if does it's a quick check and fix.
Try creating a single player game and see if the problem persists. If it does, then I'd definately try installing another gfx driver.
If everything runs seemingly smooth offline, I'd consider checking up on choke and loss.
In 1.6 choke and loss might give you the impression of freeze lags and fps drops when in fact it is related to your connection with the server your on.
Use console command net_graph 3 to check loss and choke when connected to a server. And if there are console commands for cl_updaterate and cl_cmdrate in the CS:GO console (I'm really not sure if these commands apply in CS:GO), try lowering them down by the amount of choke you experience.
IE: You have 20-30 choke, lower the rates by 20-30.