If an airplane is not stable about the pitch axis, then the balance point is too far aft. Also, you may have just too much elevator movement. When properly set up, the Slow Poke series of airplanes should be able to nicely "putt-putt" about the sky with gentle, low-stress flight. If they won't hold pitch, then they're just too tail heavy, and you'll need a bit more weight to balance the model.

Please remember that the balance point (and balance range) shown in the plans are starting points. There is nothing wrong with your having to move it wherever it needs to be moved to get the flight characteristics you desire. To our mind, it sounds like your model still needs to balance farther forward. A way to check would be to put the model into a 45-degree dive. Hold it in that line for a few seconds and then release the sticks. You want the model to pull up smoothly. If it does not, then you'll want to add more weight to the nose. If it pulls up very abruptly, then there's too much weight in the nose, but it doesn't sound like that's the difficulty.

Other things that may make the model not hold a pitch setting would be stiff controls that do not properly return to center when you release the sticks, mis-aligned surfaces, something loose, flexing control linkages, and so on. Everything needs to be checked. After checking the balance, if the model's very sensitive around the pitch axis and still won't hold a pitch setting, we'd look for too much control throw and a lack of positive centering.