2003 Pontiac Aztek Battery

Electrical problem
2003 Pontiac Aztek Automatic

I recently went out to my car and found it dead, so I changed out the battery. Now ever since I have changed the battery it keeps getting a loose connection. All I have to do is pop the hood and jiggle the red connection wire and it starts right up. The lead is connected good, but the cable has this big bulky red rubber around it so today I cleaned the connectors super good with baking soda and water, then removed that rubber stuff off the connector. Is there anything else possibly that anyone would know that would cause it to act that way.