Bitter yeasty off flavour

I have made three brews over the past couple of months that have developed a bitter yeasty flavour over time. A couple have been all grain and a couple made with pre-prepared wort. A week after kegging they are lovely beers, but then over time a bitter yeasty flavour develops. I assume that the brews are getting infected in either the fermenter (plastic) or in the kegs. In order to avoid this I have been racking into a secondary a couple of days after gravity has stabilised. I rack into a second fermenter (again plastic) and leave for a couple of weeks before chilling and kegging.

Any ideas?

Thanks Gash. CO2 is from the local brewshop, so should be ok. Kegs not cold all the time. I have limited space in my Kegerator, so some stored in my shed and I have traveled with kegs and used a chill plate and esky. But I am in Hobart so keg would not have got too warm. I sanitise beer lines when changing a keg and don't think I have got any beer in the gas lines. I have some spare line so may change over the lines and check the regulator. Other than that I guess all I can do is clean, sanitise and buy a new fermenter?

Also have thrown away old vinyl hose recently. I did find a spot of mold in some tube.....I may have used that tube, not sure.

I have been thinking about this over the weekend and may have come up with the issue. Which means I have posted in the wrong section. The last beer I brewed I brewed from a 15kg cube of wort bought from a brew shop. I poured the wort into my sanitised fermenter and added 5 litres of water, pitched the yeast and put in a fridge to ferment. The issue may be that I am on tank water and I didn't boil the water so it would have all types of nasties. Normally not an issue with all grain. Could this be the issue?

I would have used a fresh mix of Stellar San. I normally clean then mix up about 3 litres of sanitiser pour into the fermenter and slosh it around the fermenter with a cloth. Probably mix up the sanitiser a bit strong if that would make a difference. When I transferred to the secondry I would have cleaned and sanitised much in the same way. I rack with a siphon but again run sanitiser through it before and after then hang to dry.
Cheers and thank you.