The most common cause of straight lines between dots is usually when the option to record tracks is not turned off, ie, it is always active. So when the gps is turned off and on again in another location it will join the 2 points as if the path had been made. To avoid this problem turn off the option to record tracks as soon as you finish a track. In Etrex 30 it is in Setup / Tracks / Track Log / Do not record). In Montana it should be a similar setup.
The possibility would fit in the first two images. In the case of the last image, in case you have not used a boat (lol), it certainly indicates some strong anomaly in satellite tuning. If it is not hardware problem, the suggestion is to do a master reset on your gps.
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