Your exactly right. You are filling your shocks with too much oil.
AE shocks like the ones you have rely on having a small amount of air in the shock when being used.
The trick is to get them all the same.
Try putting just below the top of the shock body of fluid in the shock. Pump it and wait for all the bubbles to dissipate.
Then put a little fluid in the lid, just a small amount(!) then put the lid on.
Try to have the same amount of fluid in all your shocks.
The trick like I said is to get your shocks all acting the same.
Losi has a very good shock balancing tool which will do just that.
I hope that helps.