What would make the spark plug wire pop (blow) off sometimes only when driving.(at times not all the time). Its hard to pull off with your hands and only does it sometimes. 2rd one from left as you seat in DR seat.

 

Deteriorated spark plug o-ring. You can buy them individually or they come in a kit with the valve cover gasket.

 

Are the spark plugs tight?

 

Are the spark plug wires OEM?
I have seen aftermarkets flop off.