4WD only engages when truck is cold

My tacoma (1995 SR5) will engage into 4wd if I am sitting in reverse letting my truck warm up pretty easily, and it works excellent. But if I start to drive and disengage it for 5-10 minutes and try to go back into 4wd it will not click on. it also will sometimes engage when Im driving if I put it into 4wd and wait, anywhere from 10-30 minutes. has anyone had similiar problems? or ideas of what could be going wrong??

 

I'm not sure if a 95 uses a ADD, but my truck does and it can do some weird things sometimes. I took mine off and cleaned it up and the connectors. It seemed to work a lot better. (BTW mine was doing kinda the same thing as yours, pushed the button would take about 5-10 mins to engage)