So you think it's the speed shift that's causing this?
The undervoltage capacity in the voltage x clock curve is not homogeneous.
At some points it is possible to reduce the tension further and at others it is necessary to be more conservative.
With Speedshift at 0 the processor will keep the clock as high as possible even under light loads and this can benefit the undervolt by allowing greater undervoltage.
Generally at very low clocks (or very high clocks on CPUs with very high maximum clocks) the undervolt should be more conservative.
The higher the Speedshift value, the more the clock will vary, reaching low numbers and during the split second of switching between profiles the system becomes unstable.
As Throttlestop treats the entire curve equally, if you want a completely stable undervolt at high Speedshift values you must increase the voltage.
I usually handle this using 3 profiles.
1- High performance profile:
Speedshift at 0 and highest possible undervoltage (stable only with Speedshift at 0)
2- Balanced profile (connected): Speedshift at 84 and stable under voltage
3- Economy profile (battery): Speedshift at 153 and under voltage a little more conservative than in profile 2
When connected to power, profile 2 is selected and when on battery, profile 3.
I change profile 1 manually, always going from 2 to 1 or from 1 to 2 to avoid instability.