Erroneous flight data - NKS711 DTW - LAS on 4/9/15

A hobbyist here just educating myself on flight tracking. Saw something have not seen before which shows a dramatic decline in altitude twice for a plane. Duration and magnitude of descent seems weird once, let alone twice.

Have not come across this before. Just a data glitch?

FIRST OCCURENCE
Thu 08:42:40 PM 39.9864 -100.5390 255° West 378 435 36,000 Level FlightAware (KFNL)
Thu 08:43:10 PM 39.9607 -100.6572 255° West 377 434 36,000 -35,586 Level FlightAware (KFNL)
Thu 08:43:38 PM 39.9591 -100.6644 255° West 377 434 1,600 Descending FlightAware (KFNL)
Thu 08:45:48 PM 39.8851 -101.0028 254° West 375 432 36,000 12,900 Climbing FlightAware (KFNL)
Thu 08:46:18 PM 39.8706 -101.0676 254° West 375 432 36,000 Level FlightAware (KFNL)

SECOND OCCURENCE
Thu 08:50:21 PM 39.7501 -101.5976 250° West 381 438 36,000 Level FlightAware (KFNL)
Thu 08:50:42 PM 39.7379 -101.6401 249° West 381 438 36,000 -29,127 Level FlightAware (KAPA)
Thu 08:51:16 PM 39.7164 -101.7146 250° West 379 436 9,300 Descending FlightAware (KFNL)
Thu 08:53:27 PM 39.6383 -101.9933 250° West 379 436 36,000 10,898 Climbing FlightAware (KFNL)
Thu 08:53:43 PM 39.6285 -102.0282 250° West 381 438 36,000 Level FlightAware (KAPA)

FULL TRACK
flightaware.com/live/flight/NKS … S/tracklog

Thanks…