Joe,

Everyone here has given you great suggestions, but there is one that I almost hesitate to ask, but just for sake of completeness....

Do you go through a proper string stretching routine when you put a set of fresh strings on your guitars?

Many videos on this topic.

I assume that you do this, but just in case, this could be a source of all of your guitars having the same issue - if you are not stretching.

Honestly 300-400$ guitars these days have decent tuning machines. My Epiphone Nighthawk Custom Reissue, which now goes for only $299 at Sweetwater, came stock with some really nice Grover machines.

Do you play a heavier gauge string than what came stock on your guitars? If so, 90 dB's suggestion to check that the nut slots are wide enough and smooth enough in the very first reply is one of the first places to check. Then how about the intonation on your electrics? Do you know how to set intonation?

If not, then take the money you would spend on a set of nice tuners, and get the guitar professionally setup by someone who has a good reputation in your area. Ask him/her if they will teach you the basics on how to do that. You are a sharp guy, you can probably teach yourself given all the resources that are available today. You do need a good tuner to do a proper setup and all of the bridge and neck adjustment tools.

A full on pro setup can do wonderful things for a frustrating instrument. I would go there first after checking off string stretch and nut adjustment if necessary. Even filing the nut is something that is better left in the hands of a professional because of the specialized saw/files necessary.