Even if you use a stronger power supply, you may get the same problem along with some other problems.

Even though all V+ and GNDs will be parallel for all servos, the last ones in the power line will still be getting low voltage.

I suggest you make a connection as below.

In Figure1, S1 does not get the same voltage as S4 or S5.
In Figure2, all servos get the same voltage.

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