I have auto p5 than swaped with msp engine and tranny with msp ecu also and my cruise control work flawless. But you have some wiring to do and CC it controled by small module aside the ecu.
Good try this.....take a wire and now ground the base of the sensor where is the thread and go to engine or body. Your sensor in isolated by t adapter, hose and didn't get proper ground . Sorry for my english.
It simple fix bad clutch switch or bad neutral switch. It help feature in computer to prevent stall while declutching. Also make sure ecu see signal from neutral switch and clutch switch.