I too have been dealing with this transmission. The problem with mine may be the same as yours - low line pressure. Low line pressure is merely a symptom of a leak somewhere. According to this web site Servo Pin Bore Repair by Northland Transmission the 5R55W/S is notorious for leaking through the servo pin bore. The fix is laborious and time consuming, but not expensive. The trans must be pulled and all parts removed from the case and then the servo bore is reamed and fitted with a brass sleeve on both the overdrive (2nd and 5th gear) and mid bands. I'll post more when mine gets done.