I would say based on your latest info. that you are experiencing an intermittent loss of vacuum. This could be from a leak in the system (0r any other vacuum system that the pump is used to operate)or from the vacuum pump intermittently not operating (bad check valve on pump). The TCC system has many vacuum lines and a storage tank ( I think behind the left front fender, look for a red vacuum line coming from the vacuum fitting at the firewall going to left front fender near hinge). If you purchase a vacuum gauge/hand pump it will help you with finding this problem. You could then disconnect the lines one at a time and test them to see if one is leaking as well as see how much vacuum the pump is generating. When the TCC system quits operating, does the brake pedal get hard to push? (Like it has lost it's power assist). From the vacuum distribution block behind the glove box you can test the individual flap actuators, but you will need a vacuum schematic to see which cells you are testing so you need a manual.

Keep checking you will find it,