EMB-170 Captain reported an autothrottle malfunction during climbout. During the climb; the autothrottles began to return to a non-climb setting. The Captain disconnected the autothrottles; set power manually; and performed an air turnback.

Narrative

Departing from ZZZ; I was pilot monitoring and we were doing a flaps 4 departure. It was a normal takeoff with gear being put up and NAV called at 400 ft. At 1;000 RA; my FO (First Officer) announced VNAV. We climbed a couple hundred ft. and during this time; the Flight Director and AT (Autothrottle) gave us some very strange commands. The throttles should have been giving us full power. However; they had reduced back and were only giving us 50 to 65% power; and the Flight Director was telling us to go down in an attempt to speed up. We were of course at 1600 to 1700 ft. and roughly 140 kt. We received a Don't Sink" message as well. At this point; I took over the controls from my FO and overrode the AT in an attempt to feel what they were doing; and they were attempting to return to the previous lower throttle setting. I disconnected them and advanced them to full power and disregarded the Flight Director information. I proceeded through our normal flap retraction schedule.We alerted the controller to our situation and let them know that we wanted to return to the field. They had us level at 4;000 ft.; and we planned to return to ZZZ. They also asked if we would like to [request priority handling]; to which I said no. I disconnected all the automation and the Flight Director and got my bearings straight while responding [to] the controller's vectors. At some point; we decided it better to divert to ZZZ1; so we began building; bugging; and briefing for that. I contacted Dispatch and kept them in the loop; and also made a PA to the passengers and let them know as best as I could what was happening. Ultimately; in the simplest form; I would say the automation was giving us commands that we knew that we didn't want to do. When we input the landing data; we learned that we would be overweight for landing and that we needed to fly around to burn off fuel. I decided that the return was not pressing enough to do an overweight landing. My FO was having some difficultly setting up the MCDU (Multipurpose Control Display Unit) and since I couldn't help him as I was hand-flying; I turned the automation back on to see if it would work. It appeared to work; and satisfied with that; I began helping with the setup of the approach. To give myself more latitude and leeway; I decided it would be best to [request priority handling] for a "just in case" scenario. Dispatch then informed us that emergency vehicles would be meeting us at the plane; to which I said that would not be necessary. I asked the controller about this and they said it was standard [priority handling] protocol. The reason I didn't want the vehicles was because I didn't want to panic the passengers. I called the flight attendants at some point and asked how they were doing; informed them that no brace would be necessary; and that it would be a normal landing.Just to cover all bases here I did consider the possibility of trying to replicate the malfunction by doing some maneuvers; but ultimately decided against it as my primary concern was the wellbeing of the passengers. We burned off the necessary fuel and came in for a normal landing. The emergency vehicles did a once over; and then we proceeded to the gate. This report is being filed for a just in case scenario. I really do think that I maneuvered through this situation in a very good manner; in that I followed what I've been trained to do via not being overly reliant on the automation and overriding it when necessary; as well as keeping everyone in the loop who needed to know. I attest my performance to the excellent training that I received over the years. While I can't think of any really specific; as I've indicated to anyone that's called me so far; I'm very willing to hear any suggestions that can help me navigate through future events; small and large."