My initial WAG is chlorophenol. It tends to have a harsh, lingering bitterness that makes the beer undrinkable. That said, I can think of several other common issues that can create the taste you described.
The stalled fermentation may or may not be related. Are you also getting autolysis?
To troubleshoot, please answer:
Are you measuring pH during fermentation? Is it different when you get this issue?
Is your water source stable? Are you treating for chlorine/chloromine?
What type of yeast is giving you this issue?
How much O2 are you using?
What is your typical lag time?
Does it happen to this recipe alone? Or every recipe?
Is it related to a single fermenter, hose or other piece of equipment?
Is it related to when you get blowoff (assuming some batches don't)?
What are you using for caustic? Do you follow with an acid rinse?
How are you controlling fermentation temperature?
Are you repitching yeast? If so, which generations does this issue present itself?
If harvesting yeast, when and under what conditions?
Are you using any soft steel or galvanized equipment or fittings that contacts wort?
What is your wort chill time? Temperature?
By no means is this a complete list of questions. It's only meant to figure out which part of the process is contributing.
Making the jump from home to pro brings a lot of new stress to brewing. Feel free to PM me if you'd rather answer that way.