I have a 86 Rx7 gxl, my problem is that after I ve been driving about 40 Min or more My idle gets erratic. it goes up then drops almost to stalling.

I was thinking it was my bac valve but I didnt know if it was possible for it to be bad and only mess up occasionally.

Any inputs on my probelem would be greatly appreciated.

 

You're on the right track.
There is a BAC cleaning procedure posted here somewhere.
There is also an ECU output test.

I have to get back to work or I'd search it for you.