A close and careful reading of the engine owner's guide will explain the behavior of the engine and its alarm system. If you have the original engine owner's guide, it may explain more.
Your inference that the cause of the consistent refusal of the engine to accelerate beyond 3,000-RPM and subsequent return to idle speed is based on an electrical interaction in the engine control system does seem plausible. My basis for that is some familiarity with electronics used in that era on OMC
outboard engines. Here I am assuming the TurboJet engine is using similar technology.
The engine spark is generated by an assembly called the Power Pack. The Power Pack receives input from two temperature sensors. In the presence of an alarm input from a temperature sensor , the Power Pack will act to limit engine speed.
The engine speed limit function is called "S.L.O.W" and limits the speed of the engine to 2,500-RPM when an over-heat condition has been detected. Since you report an engine speed of 3,000-RPM as the threshold point, this function could be what you are experiencing,
Also, you have not made any mention of an aural alert sounding. It would be normal for the engine to alert the operator to an over-heat condition with an aural alert sounding going off with a sustained BEEEEEP. If you have the SystemCheck gauge, it should also be signaling visually with an annunciator lamp the onset of the alarm condition.
Let me give you a brief narrative of the alarm system wiring. The alarm sensor wiring is what we call a "wired-OR" logic, meaning that the alert sounder or annunciator will be triggered by the closing of ANY of the sensors; they are all wired in parallel. There are three sensors: fuel suction vacuum sensor, oil tank level sensor, and oil flow rate sensor. These three sensors are all connected to the TAN conductor. The aural alert sounder is also connected to the TAN conductor. If any sensor has continuity to ground, the aural alert sounder (and the visual indicator lamp) will operate.
The oil flow rate sensor is part of the OMS (Oil Mixing System) pump, previously called the VRO (Variable Ratio Oiling) pump. This sensor is designed to detect loss of oil flow, which usually shows up at higher engine speeds. It goes to ground whenever it detects an improper oil ratio. This usually produces a staccato alert when the engine speed is above idle and there is an oil flow problem. The engine speed is not limited to the S.L.O.W. function.
The oil tank level sensor is part of the oil tank, it goes to ground whenever the oil tank level is below threshold. This sensor is unique in that is produces a short to ground about every 20-seconds when its threshold is exceeded, so the aural alert is a periodic BEEP at about 20-second intervals that continues as long as the oil level is low.
The temperature sensor (or sensors, as there may be two, one on each block of a V-block engine) goes to ground when the engine block temperature is above threshold. This produces a continuous tone from the aural alert, and also imposes a limit on engine speed to about 2,500-RPM. This is the only sensor that is allowed to limit engine speed. To achieve this unique function, the sensor is wired directly into the Power Pack (via a TAN conductor) but connected to the alert sounder via a diode. In this way, then the temperature sensor operates it affects both the Power Pack and the alert sounder. The other sensors only affect the alert sounder, and are not supposed to affect the Power Pack.
A explanation for the behavior you describe could be due to a failed diode in the wiring harness. I explain further:
In the alarm system there is a vacuum sensor in the fuel line. This sensor is designed to monitor the suction pressure in the fuel hose delivering fuel to the engine, and to signal an alarm when the suction pressure (or vacuum pressure) in the fuel system exceeds the threshold of the sensor. This condition would normally occur when there was a restriction in the fuel line that was preventing the engine from getting fuel at the rate needed, and that would occur at higher throttle settings. The NORMAL wiring for this sensor is to keep it isolated from the Power Pack input (that initiates the S.L.O.W. function) by a diode. In this way when the fuel restriction condition occurs, there is only an alert produced, and the S.L.O.W condition is not triggered. However, if the diode is bad or if someone previously removed the diode, then the fuel suction sensor would affect engine speed, just as you describe. And, again, just as you described, the engine would be able to immediately resume running at higher engine speeds as soon as the fuel suction pressure dropped, would it would immediately when the throttle was reduced.
So my best guess for the cause of the described behavior: in the wiring of the alarm sensors, there is either a bad diode or a missing diode in the wiring.
Also see
WARNING HORN SIGNALS
http://continuouswave.com/whaler/reference/warningHornSignals.html