6.5 Loss of Power, Limp Mode and OBD code
 

Please help with a power and "limp" mode problem on my 1994 K2500 6.5. The SES light comes on at 71 MPH on level surfaces and I lose all power from the engine. I can push the pedal to the floor and it won't react at all. When troubleshooting the OBD1, I get a code of 78, which is a wastegate solenoid. I've replaced this with a brand new Borg Warner unit, but still get the same code and light. I've also replaced the lift pump and oil pressure switch, which had failed as well, and both are performing and testing as they should I should mention that this problem will occur much quicker on a hill, well below 71 MPH, but am unsure of the RPMs because I have no tach installed. Could this be an ECU unit? Or a MAP sensor? I suspect my problem is overboost, but not sure. Any suggestions or tests I can do to fix this? I've been troubleshooting this for a month, it is my father's truck, and he has had this limp mode problem for over 16 years. No mechanic has ever been able to fix it despite him throwing thousands of dollars into it. Truck works great as long as you don't have a hill or need to go above 70 MPH, but pretty ridiculous. Turning the engine off and back on resets the SES light and the problem, but once you hit those magic numbers, it comes right back. Any help would be greatly appreciated.