First off.. Welcome to the forum,there is a specific area to post your question,but I may be able to shed some light on the issue.
There may not be anything wrong with your head unit,
The way I see it,you likely have two problems...
First the skipping...These units are very suceptable to vibration,and you need to look at the back of the unit,to see if the plastic pin is firmly loged in the hole provided.If its anything like my 07,the radio istelf bolts up with 4 screws on the front,and a large pin that slides into a hole in the rear of the dash panel itself.If this support isnt there you can get excessive vibration,and it will cause it to skip.
Second...There are two voltage inputs to the head unit,one is switched"hot" and the other is constant hot.The constant hot leg provides memory for the clock,and pre-set radio channels.It also provided internal memory for the cd portion to resume play at start-up.
You didnt say if you were having any clock,or station memory problems,but if you are,I suspect an intermittent connection on this "hot" leg.
If you arent having any memory issues,I suggest borrowing a working one out of perhaps a friends truck,and putting it in. If the problem persists,its in the vehicle,if not I just cant believe that all 3-4 radios replaced have the same problem......Perhaps possible.If its still under warranty,keep taking it back until they either get it right,or figure out whats going on..
Perhaps posting this thread on the audio portion of this site will provide better results.