OK - according to my notes, this device has a "slit" on the left side of the unit that will allow you to add more memory capacity to the device.
I mention this because the symptoms you mentioned (trip planning issues) often occur when the internal memory starts to get full. Here is a link to where you can get these cards at a reasonable cost.
8GB microSD card SEARCH results (Amazon).
Now since I believe this is really a memory issue, you can also try doing a "master reset" of the device, which will clear all the user data (including trip memory), and should allow the trip planning to function normally again.
NOTE: You may want to BACK UP and favorites you have stored in the device BEFORE doing the Master reset, so those favorites can be restored afterward.
Here is a link to a video that shows how to do the master reset.
NOTE: This should be done when the device is fully charged but NOT connected to power.
Click Here for Video --> https://www.youtube.com/watch?v=hq8tN2SKjZE
Please give these steps a try and then TEST to see if you can plan a short trip, then a medium length trip, etc.
Please let me know what happens when you try these things or if you have any more questions.
Thanks, ***** *****