DOP is an indication of satellite geometry.  Not only do you need a minimum number of satellites, but they need to be in good positions also.  It takes 5 satellites for RTK.  You can have 5, but if they are all low on the horizon, you are going to have a high DOP.  There are times of the day when the satellite geometry is not good in all locations.  The satellites are in constant motion, so the geometry is changing continually.  The GPS antenna should be located as high on the vehicle as possible so that satellites are not blocked by the vehicle.  I will take flack over this, but on an autosteer system putting the antenna on the front is an attempt to make up for a poor system or poor calibration.  A properly calibrated quality system will work fine with the antenna located on the roof.

DOP is at its best with the satellites are high overhead and spread evenly across the sky.  They can be high overhead, but all in the same general direction.  That will give a high DOP also.

Trimble has planning software on their web site that you can download to run on your desktop computer.  You also have to download the latest "almanac".  This software will give you the predicted satellite condition at your location through out the day.  It will tell you the times of day that will likely give you problems.