I don't believe in down regulation, but I do belive that your body tries to find homeostasis. Lets face it. We both know that you can't take X amount of gear and continue to gain 2 lbs a wk, for the rest of your life. Assuming you're eating more to accommodate the muscle you're gaining along the way on a cycle, there is still going to come a point that you either need to A, up the dose, B, change compounds, or C take a break from gear. You just get burnt out over time. There is only so much gear you can add and so many tricks you can pull before it just isn't going to be effective. At some point, we all need a break, even if your labs are good.
Whatever is going on, something is no longer working. It may be the gear. It may be diet. It may be training. Remember, what worked before may not work now. Maybe you need to adjust your training, to take it to the next level. The more advanced you are, the more imortant your training becomes. I have found that for me, changing training has played the biggest role in the last few yrs. Better training may not always mean heavier. It might mean a different approach. Maybe its more reps? Maybe its more rest between training sessions? Less or more sets?
I don't know what your gear experience is like, but maybe it's time to add more gear? Or peptides? Growth and insulin, if you aren't already. These can be game changers when you have tapped out on gear alone.
As for your cruise periods, if you are feeling burnt out, make sure you are on a low dose during your off time. HRT is lower than a lot of guys think. Everyone says 200mg EW. Thats the higher end, IMO. I do know some guys that are given 300mg EW by their Dr, but I know a lot more guys that use lower doses like 100mg EW, or even every 10 days. Some guys come off test all together and cruise on HCG. Whatever you do when you cruise, make sure it isn't just lower dose cycling.