RPM dropping at 1500 ??

RPM drop while driving
I recently did some work on my truck, new fuel filter, oil, oil filter, valve cover gaskets, spark plug tube gaskets, intake mainfold gaskets, MAF sensor cleaned, throttle body cleaned, and cleaned some dust and dirt off the air filter. About a week ago it started to drop rpm while I drive once i get to speed and the rpms come to the normal 1500-1600. They drop down to 1000-1100 and i can feel it aswell as watch the rpm gauge drop. I am getting no DTC codes or check engine light. I am stuck and have no idea what to check next. Maybe PCV valve? Thanks for any help!

 

This might be of no help, but make sure all the connections are correctly fitted under the hood. Once after a spark plug change, I left some vacuum hose or something off. I even looked dead at it in the write up and was warned not to do that. Did you do all the work yourself? Is everything installed correctly? Something like an improperly seated gasket doesn't seem like it would produce the problem you're describing, though.

 

I have looked over every plug, wire, vacuum line and anything else and cant seem to figure out what is wrong!!