I would use the 'modify' checkbox instead of the slider and enter values directly into Register00 using only values like 08,10,18,20,28,30,38,40,48,50,58,60,68,70,78 etc. There is a slight flaw in the slider routine which can result in the wrong value being sent to Register00, I don't know if this has been fixed in the latest DMT but the author was made aware of the problem.
If you want to try and be stable at approx 5120kbps, you could try putting in various high values (during the daytime when the DSL signal should be strongest) until you sync near or at that speed, then pray it stays like that all the time, lol. A lot depends on if your SNR margin drops at night and what your Target SNR margin is set to in the DSLAM (higher the better if you have a noisy line).
I don't know what your 546 is like for generating heat but my 516v6 is like a mini heater, convinced this can't be good, I've put heatsinks on the 6338 chipset, RAM and Line Driver IC, plus cut a hole in the top of the case and inserted a small 12v fan

Pretty sure it's helped stability and reduced CRC/HEC errors but I'll find out for sure over the next few days.
For true stability on MAX you'd need a router that did Tone Masking or allows you to set a maximum sync bitrate, there's a few around that do that kind of thing.