I’d suggest two things:
1. there should be a switch on the bottom for cruise control - try toggling this and running without it. I imagine your speed will be restored
2. is there a chance this loco could have had its speed steps adjusted via TMCC at one point? Klines speed control had up to 256 speed steps. I had a set of their F3s that exhibited similar behavior after accidentally being set to this
The k-line EP-5 is a beautiful loco - I hope you can get it back to running order.