As above. Our VDSL connection has been a little strange with changing DLM profiles. It started on DLM-1 (1ms), giving us the lowest possible latency, much like being on an un-interleaved ADSL connection. However, a few weeks later, the modem reset and the latency increased by about 16ms, probably bumping us up to DLM-2. It has remained at DLM-2 for quite a while now. Although, the other day our profile did drop back down to DLM-1 for 4 days, then last night, the modem reset once again and it shifted back up to DLM-2. I'm not too sure why it won't just stay on DLM-1? It's been 100% stable while on DLM-1, and the decreased ping is something I really appreciate. 

Our modem is generally very stable, no random resets except for these DLM profile swaps. Speed has been the same on all DLM profiles too, so shifting to a higher latency profile for speed benefits isn't the case at all. I'm not sure if there is anything the ISP can do, but as far as I can tell its out of my hands.