I think utilitarians are usually sincere, and most failures to live up to their beliefs can be explained by akrasia.

If all you mean by "sincere" is not explicitly thinking of something as deceptive, that seems right to me, but if "sincere" is supposed to mean "thoughts and actions can be well-predicted by utilitarianism" I disagree. Utilitarian arguments get selectively invoked and special exceptions made in response to typical moral sentiments, political alignments, personal and tribal loyalties, and so forth.

I would say similar things about religious accounts of morality. Many people claim to buy Christian or Muslim or Buddhist ethics, but the explanatory power coming from these, as opposed to other cultural, local, and personal factors, seems limited.

One of the most effective ways to signal you are altruistic and trustworthy is to actually be altruistic and trustworthy.

I agree with the main thrust of the argument, but such signaling would only apply to potential trading partners to whom you make a habit of speaking openly and honestly about your motives, or who are unusually clever and perceptive, or both.

If you want a plausabile theory as to how natural selection could produce sincere altruism, look at it from a game-theoretic perspective. People who could plausibly signal altruism and trustworthiness would get huge evolutionary gains because they could attract trading partners more easily. One of the more effective ways to signal that you possess a trait is to actually possess it. One of the most effective ways to signal you are altruistic and trustworthy is to actually be altruistic and trustworthy. So it's plausible that humans evolved to be genuinely nice, trustworthy, and altruistic, probably because the evolutionary gains from getting trade partners to trust them outweighed the evolutionary losses from sacrificing for others.

Altruism - at least in biology - normally means taking an inclusive fitness hit for the sake of others - e.g. see the definition of Trivers (1971), which reads:

Altruistic behavior can be defined as behavior that benefits another organism, not closely related, while being apparently detrimental to the organism performing the behavior, benefit and detriment being defined in terms of contribution to inclusive fitness

Proposing that altruism benefits the donor just means that you aren't talking about genuine altruism at all, but "fake" altruism - i.e. genetic selfishness going a fancy name. Such "fake" altruism is easy to explain. The puzzle in biology is to do with genuine altruism.

the way you have written this makes it sound like you think utilitarians are cynically pretending to believe in utilitarianism to look good to others, but don't really believe it in their heart of hearts. I don't think this is true in most cases, I think utilitarians are usually sincere, and most failures to live up to their beliefs can be explained by akrasia.

So: I am most interested in explaining behaviour. In this case, I think virtue signalling is pretty clearly the best fit. You are talking about conscious motives. These are challenging to investigate experimentally. You can ask people - but self-reporting is notoriously unreliable. Speculations about conscious motives are less interesting to me.