I thought I replied to this already, but I must not have...anyways....
What you are experiencing isn't that uncommon. GPS at low speeds is not very accurate at all and will jump around on you. The common remedy to this is to use a water speed sensor. Generally speaking, these are much more accurate at low speeds but very inaccurate at high speeds. This is where GPS will be the best. The water speed paddle wheels can be effected by things such as wave action and current so you can run into similar situations with that as well.
-Lawren
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