If you're looking for something that won't aromatize, and will give you lean muscle mass, try Masteron E, or Primobolan. You can use EQ, but it still aromatizes, and you will need an AI. Your cardio may be slowing down due to excessive water retention due to testosterone conversion into estrogen. I use Test E, but very sparingly, and I always use an AI, and I also take another OTC product called DIM, it will turn your estrogen metabolic pathway on and help you get rid of excessive estrogen levels, leading to less water retention and increased cardio output, while protecting your HPTA. Try your AI's at night, and run the DIM in the day, twice a day at 500mg per dose if you continue to run high levels of Test E. 1,200 mg's a week bro, that's high, less is more. If you're trying to build mass, run some Deca, 200 - 300 mg's a week with 600 mg's a week of Test E, (or every 5 days ) remember twice the amount of Test as Deca. What specifically are your goals as far as gaining mass, or just increasing cardio?