Tach reads less than 500 RPM at idle

[jraker]

My 06 megacab automatic stock has 60k miles on it. I recently changed the transmission fluid and have noticed the tach behaving differently in the last 2k miles. It used to idle ~650-700 RPM and now it is below 500. When I touch the accelerator it shouts up. It is not always that it does this and I have tried to pull codes and haven't seen anything come through on the odometer. The condition it does it in is stop and go traffic (sporatically) and coasting down hills at any speed. It doesn't feel like the engine is bogging down however I have noticed a stumble when in cruise control going down hills which is new. I put a set of gauges on the truck when I first bought it two years ago at 18k miles. The transmission runs around 160-170 in the summer (90*) in stop and go traffic. Right now it is lucky to see 120* (40*). It hasn't been modified by me and I don't believe the last owner had a programmer on it either. I can't find evidence of one and the guy barely drove it. Any ideas? Torque Converter locked or going out?

 

[seafish]

2006 gauge clusters are notorious for the tach not reading accurately due to failing solder joints on the back of the cluster.
There are threads that detail HOW to remove and fix it, if you are elctronically inclined. It is MUCH cheaper then buying a new gauge cluster.