[ddruker]

The flashing D4 (if your gear selector is in drive) or P4 (if your gear selector is in park) indicate the transmission has detected a problem and has put itself in limp mode. When this happens, the transmission computer should contain log information that will tell you why it put itself in limp mode. As you have noted, when you clear codes on your ECM it will also clear the transmission error temporarily. What sounds like has not yet happened is that someone has actually read the transmission error log. You can't do this with a standard consumer grade OBDII code reader, you need a more advanced reader that knows how to talk to the automatic transmission. If I misinterpret your note, any mechanic was able to read the transmission codes, not just the OBDII engine codes, I apologize in advance.

The transmission oil cooler switchover valve being talked about above fails into an open state, meaning your transmission coolant will be going through the cooler even when the engine and transmission are cold if the valve is failed. So I don't see how that could be leading to problems that occur after driving a hundred miles - when the valve fails what happens is your transmission doesn't warm up as fast as it normally does.

The transmission coolant change over valve is a $30 part, but it is a big pain to get to. I replaced mine when I took the transmission out and did a rear main seal and IMS bearing replacement. It had been failed probably for years and didn't cause any problems.

There would be two common problems that lead to this valve failing. One is the overall vacuum system which feeds this valve and also feeds your secondary air injection system. Since you were Boxster isn't reporting any SAI problems, it implies your vacuum system is in good shape. The second problem is that this valve itself has a rubber membrane which will disintegrate over the years. When this happens the valve won't move and just sits in an open status no matter what. That's how mine was. As I said above I don't really see how this could lead to any serious problems.

If it were me, I would change the fluid and the filter and see if the problem went away after that. If not, I would ask your dealer for a recommendation for a transmission shop that is familiar with Boxsters. Some mechanics may not be very familiar with ZF transmissions, and your mechanic needs a more advanced code reader to read the transmission logs to really understand what is going on. A transmission shop should have no problem diagnosing and fixing this problem.

Hopefully you'll get the bottom of this problem soon and get back to having fun driving.