Keeps rebooting after screen and powerbutton flexcable replacement

Where did you buy the replacement power flex? Did the part # stamped on your part match the original? Does the top sleep/wake button depress (click) at all? If no, you likely installed a lousy aftermarket part where the power switch (under the button) is too tall, and therefore will remain pressed down after you tighten the two screws that attaches the switch to the small plastic bracket on the top of the phone. If yes, you still may have a bad power flex, as I have ordered this particular part from about 7 ebay sellers to date, and have found NONE that actually fit/function on the Verizon 4/4S's reliably.

Some guys will try to "make it fit" by backing the 2 screws out until the bracket flops around and/or the screws come out and toss around inside the phone, risking a short! I have yet to find a reliable source for this part that is not an "Official Apple OEM" part! Please let me know if you do!