Hi Scotty! I have a 2005 Trailblazer 4x4 4.2L I6. Just turned 230,000 miles. On the highway, between 1500 and 2000 RPM, the truck shudders periodically. If it shudders and I give it a little more gas, the shudder stops and then comes back when the RPMs go back down. If I drive in 3rd on the highway it won't shudder and everything is great in town. What can I do to fix this?