If this was with a DVM, then it's meaningless. You can only look at data with an oscilloscope or logic analyzer. (I had to add the logic analyzer even though only one in a hundred experimenters would have one, but if I didn't, someone would jump on me with "..and a logic analyzer").
Not a power issue?? Prove it. Remove any one string from the circuit. If the flicker stops, then you do have a power issue.
I don't put strips' data lines in parallel, I just drive them with another data pin on the processor board. Yes, it's more coding but it gives you the flexibility to make each strip unique, or alike.