Hi SoonerChris,
I’m guessing you are referring to a mattress protector rather than a mattress cover (which is part of the mattress itself).
There is more about the pros and cons of the different types of mattress protectors and some examples of each of them in post #89 here.
I would also keep in mind that some people tend to sleep warmer on memory foam mattresses so the memory foam itself may be part of your temperature regulation issues as well.
While it’s not always possible to to track down temperature regulation issues for any particular person on a specific mattress because there are so many variables involved (including your room temperature and humidity, your sheets and bedding and bedclothes, your mattress protector or any mattress pads you are using, and where you are in the “oven to iceberg” range) and some people can sleep warmer on mattresses that most people are generally fine with … there is more about tracking down a potential cause or causes for temperature regulation issues (at least to the degree possible for a specific mattress) in post #2 here and the posts it links to that may be helpful.
Phoenix