Hi, Lyle.

The tire size issue we discussed previously causes the cruise control to disengage (or fail to engage) because the traction control system perceives the difference in rolling diameter as a loss of traction when the resulting difference in rotational velocity reaches a certain point. The problem you're describing sounds similar, but with properly inflated stock size tires on stock size rims, this shouldn't be the case. I suggest that you take a look at your traction control system, including wheelspeed sensors, to make sure everything there is in order. One question: does the antilock braking system function properly?

Best Regards,
Rob