This is an older post...but I hate reading things and not finding solutions. I have a solution to this dilemma.
The issue is this - Fuel pump not priming with chipped ECU.
My issue was also with a P28...but the ECU doesn't matter. It's your chip itself or the ZIF socket you are using.
When you put your chip and/or ZIF socket in the main socket, you most likely bent the legs of the chip / ZIF socket in a little bit to get it to fit better. When you did this, you bent them in just a little too far. This is causing an intermitent loose connection and you may not get a CEL when you turn the key to the "ON" position, but the fuel pump won't prime. Usually until it accidentally makes a connection again.
Remove the chip and/or ZIF socket and bend the legs back out a little. Then carefully push it back in. Make sure that it takes a little bit of force to get it back in the main socket. This ensures a proper connection.
Also, take the chip or ZIF and look at it sideways and from the bottom. Make sure that the legs are even across each side. You can use a table or any flat top / edge to put a little pressure on them to even them out.
In my case, It was my ZIF that was losing intermittent connection. Since I am done tuning and swapping chips all the time, I just removed the ZIF altogether and pushed the chip into the main socket.
At first, this seemed like it might be a wiring issue or maybe the ECU going bad...but as usual, the simplest solution was the right one. My car hasn't done that since I did the above procedure.