I thought maybe this was just an anomaly, but now it's happened twice on my '98 ML320

Both times were after the car sat at work all day, and was happened after I started it to go home.

Normally the car shifts from gear to gear around 2200 rpm or so if I'm not stomping the gas.

But during these two times, the car never shifted until well above 3000-3800 rpm even though I was driving lightly

I got to about 50 mph, the car was still driving at around 3500 rpm (unlike around 2000 rpm when cruising at 50).

At a stop light shifting into neutral and back into 5 did not solve the problem.

Finally stopping the car, and restarting the car corrected the shift points.

Is this a bad sign? Is this a known problem?

Thanks,
Tim