How to sterilize CO2 lines and How often

I have had many great beers and then a bad one, off and on for a year or so, and attributed it to my brewing.

After cold crashing my beer, I usually put some beer into some 32oz plastic containers and force carb with those blue carb tops. The beers taste great, then a week later when the keg is carbonated I get a slight slippery feel or taste and I have lost all the clear - crisp taste of my IPA's.

Did this again last week, new beer is a Citra/Simcoe, and tasted great in the 32oz within 2 hrs of pulling out of the cold crash.

A week later = might just pitch it,

Kegs are cleaned with oxyclean, rinsed 2x with Sanitizer, purged with CO2, no splash, topped, filled with CO2 and released 3-4 times.

Think I might have hit on the issue:
when force carbing I occasionally build up more pressure in the 32oz container and a little beer goes up the CO2 supply line.

Thinking it goes bad? Cant think how, there should be all CO2 and no O2 but I am stumped,

OR,

its the same keg that keeps going bad - I will mark it this time.

So, should I clean out the supply lines and with what?