Transmission Slippage 2012 Ford Econoline

I have a 2012 Ford Econoline,  it came in for a transmission issue, when driving it would slip out of second gear, checked the fluid, fluid was burnt.  We then proceeded to change to transmission itself.  After it was replaced, test drove it and the transmission seems to shift fine, but when you fully depress the gas peddle, it seems to slip out of gear, the RPM will max out and the vehicle will not speed up. I then let off the gas, and the transmission will kick back in.  I have replaced the TPS, replaced the Spark Plugs, checked fuel pressure, cleaned the Throttle Body, replaced the air filter and it still has the same problem.  Any ideas?  Please Help