Hey Aetjansen,
I've been struggling with the same problem as you are (were). I'm wiring 10 TLC's, and with me they also start giving me problems when more than 5 are connected, so I'm really glad i found this post. Sadly my electronic skills are quite bad, so I'm having some problems with translating the de-coupling tutorial into what i should do. Could you tell me between which pins (of the TLC's) exactly you connected the capacitors?
Thanks in advance.