My 2006 3500 steering wheel sticks intermittently. I was driving at 65mph and the steering wheel appeared to stick. The wheel would pop loose when I applied pressure. I pulled over and did a quick mechanical check and didn't find anything. When I looked at the P. S pump I noticed the cap was gone. I made a quick cap out of some tape and a bottle cap and made it home with no further problems. I bought a new cap and a week later it happened again. The cap was still in place and when I removed the cap it appeared to be under pressure. I'm not sure if I have air in the lines, a pump going bad or something else going out. Any suggestions would be appreciated.