Often surplus Military and Police firearms have cosmolene or dried grease, oil, and gunk in the action. I had an Israeli Police trade in P35 Hi-power (Hungarian contract) that had similar issues. There was dried gunk in it. I used spray brake cleaner to cut the crud. Field strip, and do this to slide and frame without grips. Spray it well, let soak, then sprayed again and use a soft tooth brush. Spray again, soak, and use air to blow it out.
Also, if you know a mechanic, their parts degreasing sinks work really well, as does letting it soak in a stainless steel pan of warm kerosene or diesel. Brush and blow dry.