The one thing to check that is in the Accumulator valve body is the Modulator plunger size. Having a broken spring or an oversized plunger could cause this kind of shifting.
.372" for this plunger for big V-8 and Diesel.
.472 O.D. High performance
.500" shift hard racing applications.
Pull out filter (pushes in to bottom of pump cavity;
remove Accumulator Valve Body;
Remove pull clip on modulator valve;
Remove Accumulator Sleve and Plunger;
(Note each way spings and inner plunger are installed)
Mic O.D. of plunger and replace if necessary.
If you need assistance, I have pictures of a rebuild. Just let me know and I will send you pics.
As far as the Line Pressure Regulator, the actual valve is located in the pump.
Did you check the line pressure at idle and then elevated RPM's during shifting to see if it was correct?
Hope this is helpful.
If you need help with overhauling your own E4OD, I am producing an ebook that can help you build tools and rebuild the E4OD trans.