Although the TV cable could be all or part of your problem, the 700R4 transmissions are known for excessive 4-3-4 shift cycling at ~1/2 throttle and TCC disengagement/engagement cycling.
Absolutely verify TV adjustment first. Then...
TCC is disengaged with brake application. If the brake switch is right at the edge of its adjustment, pedal vibration at cruise can cause the switch to make and break contact and TCC will drive you nuts disengaging/engaging. Same thing can happen if a driver hovers the brake pedal with a foot, using it like an ottoman. Additionally, the brake switch can be easily knocked out of adjustment if a driver catches a foot under the pedal by accident and lifts slightly.
To determine if the TCC is the problem, try this: at road speed, slightly depress the brake pedal, but not enough to apply the brake. You should "feel" the TCC disengage. Releasing the brake pedal you should "feel" the TCC engage. Sometimes it's subtle - helps if you have a tach. TCC disengagement/engagement should occur within ~1 second of depressing/releasing the brake pedal. With the TCC forced to disengage by applying the brake as described, do your symptoms go away? If so, adjust the TCC switch (located next to the brake switch under the dash).
If the problem persists after defeating TCC as described above, and you verify that the trans is actually dropping out of OD, you may need to upgrade to the Corvette 3-4 throttle valve sleeve...
http://www.summitracing.com/parts/bmm-70237
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