So one thing, if you used the periodized 5x5 and saw results right from the start...chances are you didn't really need a periodized program and were benefiting from increased frequency and workload on the core lifts (i.e. you might have gotten something from the rebound too but if you made linear progress during the first 4 weeks, this is the case). I have the periodized 5x5 set up for fairly low volume (i.e. like early periodization programs) so that people will still get results, not kill themselves, and can come from a bodybuilding background as long as they know the core exercises and have been training them for a while. This segways into DFHT.
DFHT, has gone through a few iterations and I think each one was to lower the volume and make it more accessible. Lots of guys have run it and done well and it was designed with feedback and input from some really sharp people. A number of people have also found it just too damn much and had to change it to suit their own tolerances - in contrast to the person who designed DFHT, those he consulted with, and the people who ran it, these people had a BBing background and hadn't been exposed to many programs like this before. Unfortunately, the issue is that workload and tolerance are very specific to experience and the individual. What is deloading for one person might be enough to kill another. Main issue was that the people using it were very experienced lifters with a lot of experience and some major capacity, as it trickled down it had to be scaled further and further back.
Takeaway, the base template you used is too much volume for you. Not sure if you have the word doc from my site but I think that was the original which means some people might find it a monster. This is sort of why 'cookie cutter' programs rarely get good results accross a broad range of people and why almost none of them vary any aspect of volume or workload other than weight lifted (i.e. frequency and volume are static for each person); that stuff makes it very individual and the hope is that the least common denominator will be good enough to work for most and the others deal with suboptimal results.
So anyway, this is why I always make the point of learning about your tolerances. You know the base 5x5 works okay and now you know DFHT is too much for you (believe me, I've been in the same boat before using a program from someone else and I was dead in a couple of weeks). So what you might do with a DFHT template is scale it back. The model is sound though, just too much loading for you to recover from and bounce back. The workouts Pendlay puts his more experienced lifters through would likely kill you also, nevertheless this doesn't mean the program is bad (he consistently gets excellent results) it's just inappropriate for you (a given individual at this point in time) based mainly on your past training experience and to a degree genetic makeup and psychology can play a part. Maybe see if the one posted at the online Core Magazine site is less volume for comparison.