Hesitation/shifting probem in '99 Blazer

My '99 Blazer with 4.3 and Automatic with 130,000 is hesitating. When the scanner is hooked up while driving it reads that the engine is shifting
from 1st, 2nd, 3rd, and 4th in the first couple of seconds basically before the engine ever gets to 1500 RPM's. So it acts like a hesitation problem though the actual problem is that it is shifting way to early. This happens about 50% of the time. Everything is fine if I manually shift the vehicle.
SES light is on and the code is "secondary air injection". My mechanic doesn't think this is related.
Recently changed plug wires, cap, rotor, fuel filter. Checked the EGR valve, and found and reattached a disconnected vacuum line though nothing has fixed the problem
Any help would be great. Thank you, Adam Helmer
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