2001 Acura CL Slipping Gear

I just bought this car as used and soon this problem is occurring.
This car has 5 speed and also has both of manual and automatic transmission. 8 out of 10 times the gear gets slipping right around 30-40 mile (believe it is gear#3) no matter it is on automatic or manual mode. I can feel the engine starts dragging/pulling heavily before it starts slipping then becomes normal after 40 mile or so. The TCS light also comes on occasionally when this happen.