From your description, Turbo boost is operating as it should. Or I should say, as Intel intended it to work. There is nothing wrong with your PC, and nothing you can fix.
Turbo boost, increasing the speed of your processors cores for better performance, is meant to take advantage of the situation (simplified for this description) where the CPU temperature is below a certain level, and other factors are such that they allow the processor to be operated above its normal operating speed. After operating for a while at the higher speed, the CPU temperature increases, and the other factors change to the point where the CPU can no longer maintain the higher speed without potential damage, and it slows down a bit again. This process can repeat again as you have seen, once the initial conditions that allow Turbo boost to occur are again established.
Turbo boost is controlled mainly by the CPU itself, and also by the BIOS. You won't find it in anywhere in Windows. Depending on the mother board and its BIOS, users may have some control over Turbo parameters. On laptops, those options are not normally available, due to the cooling and power limitations that are intrinsic to laptops. Even if they were, the CPU would overheat and slow itself down more than it's normal operating speed, or even shut off.
Manufactures of desktop mother boards for PC builders have learned how to set certain data items that Turbo uses to control its operation, such that the Turbo boost speed will be maintained indefinitely. The people that buy those boards also use expensive CPU coolers, PC cases with enhanced ventilation, and use high wattage power supplies. But those PCs are large and heavy, use a great deal of power, and cannot be moved easily. If the CPU becomes to hot in those PCs, it will also slow down or shut off.
Those are the tradeoffs between desktop and mobile PCs, they are not equivalent in performance or portability. You can have one or the other, but not both, yet.
If you are comparing your laptop's Turbo boost with that of a high performance desktop PC, you will be disappointed. A laptop has neither the cooling ability, nor the power available to compete with it.
The best you can do is make sure your laptop has good ventilation, nothing blocking the air intake slots on the bottom, and keep them free of dust and dirt. You can buy special laptop stands that have fans built into them for extra cooling, that may allow Turbo speeds to last longer on your PC.