Programmable Electronic Load Design
This is a suboptimal circuit for the application. You shouldn't be trying to switch the load transistor on and off in time with the pulses &mdash; that's just silly.

Instead, you should be using a precision peak-detector circuit on the feedback from the sense resistor, so that the circuit only attempts to regulate the peak current. This eliminates all transient effects inside the load control circuit.

In other words:


Eliminate that entire upper circuit that runs from PlcOut to Feedbk
Replace R9 with the peak detector.