Robert, There should not be any significant difference if you develop your film at somewhat higher temperatures. Popular belief suggests a slight quality loss--less smooth tonality and increased grain--but in practical terms, if you process at 75 degrees instead of 68, you should not have any problems. Naturally, you have to make the appropriate adjustment in processing times. I use PMK developer in the summer at temps as high as 80 degrees with no apparent repercussions.(posted 8406 days ago)My impression, from what you described in your email, is that the cause of your problems may be attributed to old or exhaused fixer. Everything else you described sounds fine to me, so that is where I would look. Try mixing up a new batch of fixer and see if that doesn't correct the problem.
Lastly, I wonder why you've chosen Tri-X Pro? There are so many superior films, I wonder about that choice. Also, while the venerable HC-110 is still a propular developer, there are many choices which produce superior results.
The most propular developer among fine art photographer these days is PMK. I've used it extensively with formats from 35mm to 4x5 and I'm still astounded by the combination of tonality, acutance and fine grain. It's a bit more troublesome to use than the typical commercial developers, but well worth the effort. You can buy PMK from The Photographer's Formulary (http://www.montana.com/formulary/). They sell it in liquid and power kit forms. You should also buy their alkaline fixer, TF-4 if you use PMK. If you want to save money, you can find the PMK formula on the web (http://unblinkingeye.com/) and order the necessary chemicals to mix your own. It is very inexpensive to use--$50 in chemicals should last you years.
As for films, it's tough to beat Ilford's HP5+. Rate it 250-320 and develop it for 12-13 minutes at 70 degrees in PMK. You'll be joyously impressed, I assure you. Ted Kaufman