A common problem in loading the data into forms is mismatch in speed with which data is send to the target application. i.e. Data Loader sends the data at faster speed then the target application can process it.
So a better way to overcome this problem is defining delays. You can insert Delay Columns to wait for some milliseconds before processing the next cells. The default value for delay column is 50 milliseconds, you can adjust the default value in options window. A delay cell is identified by a "#" in its first position.
You can also define delays for shortcuts which you are using in the Define Delays window as shown below.
When you define any Delays in this Window, Data Loader will wait for this much of time after processing the Shortcut keystrokes before processing the next cell.