The S means that your scanner is breaking squelch (receiving a signal). The T means that the signal you are getting is understood as trunking data for the type of system that TSYS is programmed for.
The "T" shouldn't - usually - show when someone is talking, because you are receiving voice, not trunking (control channel) data. On some systems where control data is communicated along with the voice traffic, you might see the T flicker or show steady during voice.
My suspicion is that your trunking tables are messed up for the TSYS in question. That's about the only reason I can think of that you wouldn't get the T while parked on the wildcard, but would get good decode when you go in to analyze mode.
Although I'm far, far away from you and the system in question, I could take a look at the programming and see if I can spot anything strange. Are you able to download it into either a PSREdit or Win500 data file and either post it here as an attachment (zipped up) or email it to me at
jay911@gmail.com?