I've been having this problem for a while now but it's becoming more frequent. It's never happened when the tranny is cold but sometimes when I've been idling somewhere for a while and then take off again it won't shift into third gear. Once the tranny is totally cool again it works again, at least for a while.
Anybody got any ideas?

 

I would guess the 3rd gear solenoid on the VB is going bad. As it gets warm it will start to misfunction.

 

Does the VB have to be remove to change the solenoid? Any recommendations on where to look for one?

 

Start with ATS and see if they will tell you what they used for solenoids and what they think.

That VB is a one off unit and I have not seen what they used for electronics. It could be a common solenoid from other trans applications or a specialized one.