What's going on with sudden acceleration/deceleration in mid-air? I was recently on a flight between LGB and PHX. I've been flying since I was 5, am now 31. After takeoff, as the plane approached cruising altitude, the plane slowed down. A lot. Then accelerated again, followed by another slowing down. This happened at least 4-5 times during the short flight, and before landing we made a complete circle around the airport, I'm assuming to comply with which runway to land on. My question is: why, or what cause, does a plane slow down in mid flight where you move forward in your seat, like hitting the brakes in your car too hard multiple times. I understand training and all that, I want to know the cause. To put it in perspective, I assumed the engines stopped. And to increase the nausea, we were delayed by a mechanical issue for an hour.
Without knowing the specifics of the flight, there could be a lot of reasons for this.

One guess for the sensation of accelerating and decelerating We’re intermittent steps at fixes on the departure procedure the pilot flew out of KLGB at certain altitudes and airspeed either as part of the DP or as assigned by ATC in order to facilitate departure from SoCal airspace to enroute.  That and a little sloppy airmanship on the part of the flight crew.

As for the “circling” at KPHX,  most likely that was the pilot flying a STAR with ATC final vectoring to a visual approach or SIAP for runways 25L/25R/26 at Sky Harbor Intl.