1987 t2. At 3800 rpm the turbo tries to engage but want. If you try and give it gas without the turbo engaging. It will rev fine. Have done a lot of work new tank, pump, lines, fuel filter, oil filter. Spark plugs, wires. Few other things. Doesn't over heat.

 

Clogged or cracked secondary injectors possibly?

 

Confirm fuel pressure and start inspecting your secondary injectors and the wiring to them.