Acetaldehyde is usually a sign of poor fermentation health, so if you haven't tried a yeast nutrient, and more oxygen at pitch yet, I would recommend those.
Another thing to check is the calibration of your fermentation thermometer. If it's reading too warm, you could be fermenting colder than you intend and not helping your yeast to finish strong.
Are you doing anything to measure your wort pH and changing water chemistry?
If your pH is crazy high or low it could cause your yeast health to suffer.
Can you also post some of your OG, yeast strain, fermentation temps, for the bad batches?
Last 2 batches I've not oxygenated the wort. My supply of O2 I thought I had were both empty.
I use a tilt and calibrated the temp and gravity before this last fermentation. Gravity (and likely temp) were both off the previous batch.
I'm not measuring wort ph. I rely on brewers friend water chemistry calculator. I believe I have been getting good results on water chemistry. However, this last batch, thanks to the apartment brewer, I just did spring water and a couple of milliliters of lactic acid. I think I'll go back to adding the brewing salts like before but I don't think that's the issue here as it's persisted through multiple brews.
Kolsch (1.048-1.012) Kolsch yeast at 62 for nearly two weeks then slowly ramped to 68. This brew was fantastic at first but competition said it was estery which I did start to notice
Festbier (1.056- 1.009) 34/70 at 52. After several days ramped up to 56. Then every few days I bumped it a few more degrees until rest at 68 for 2 days then cold crash. Beer was ok at first but had that green apple ester then tasted vinegary after awhile
American Lager (corn) (1.047- 1.012) 34/70 pretty much same fermentation as the festbier. Apple ester and then I threw out after a couple of weeks of no changes
Cold IPA (1.061 - 1.013) 34/70 started at 62 and reached final gravity in 4 days. Ramped up to 65 and held for a few then cold crashed. Only kept this one in the fermenter for about 10 days. Don't think that's a good idea with this yeast. Same apple ester but stronger. Beer would have been lovely if not for it. Threw out.
CHANGED ALL COLD SIDE 'ROUGH SURFACES'
American Lager (Rice) (1.061 - 1.013) 34/70 No clue how the SG was so high. Guess 2 lbs of rice will do that. Only pitched 1 34/70 because I was expecting low gravity. 52 for about 4 days then bumped to 55 until about day 18. Cold crashed and kegged a few days later. Also this was first brew without adjusting water chemistry aside from lactic acid. Apple esters and acidity.
Important to note that all these brews tasted great coming out of the fermenter right at kegging EXCEPT the last batch. However, that tasted great about day 12. 6 days later it tasted of strong overpowering alcohol before kegging. That's why I changed my equipment because I thought there was an infection somewhere.