Cranked up to "full" on my 590 to grill some chops. Took a long time to get up and went up and down 40-50 points during the hour I spent watching.  I tested with my Maverick external temp probe, and it was a nice steady rise. Reached 510* on the Maverick in about 35 minutes (ambient temp was 60*).  So after it cooled down, I noted that the control temp probe had a fine layer of ash and underneath that was covered with a shiny coat of creosote. I had forgotten to clean it after the last two cooks.  So now it has been ScotchBrited and is sparkling.  Just an FYI.  Don't forget to clean your temp control probe.  I suspect that the pit was fine, just a dirty sensor.  ?? Anyone else had this problem?  Also, I've been using Lumberjack Hickory Blend, and while it has less ash, it still is enough to layer the upper chamber. Will report if that resolved the problem.   

I've haven't cleaned my temp probe in 5 years and it hasn't been an issue. How could a thin layer of ash or creosote cause a problem?

 When I had my Traeger, I scraped the ceiling to find anything that would tame temperature fluctuations, cleaning the RTD being one of them. The end result - I found it made no difference it it were shiny or

 had a build up on it.

 Later after I got my Memphis Pro, I was in contact with Memphis with questions, a lot were carryover problems or quirks I had had with the Traeger..One was about keeping the RTD clean. Their response was

 they had done testing, clean vs dirty and found no difference. Their advice was to give the RTD a gentle wipe @ clean up, and not to use any kind of chemical cleaner on it..I adhered to their advice resulting in

 the last 3 years of spot on temperature readings and normal heat up times..Obviously, heat soak times will be longer in colder temps for any pit.

 Thinking about it, IF the Rtd was HEAVILY built up with crud, it would act as an insulator only slowing heat transfer to the internal coil. It would have to be a rather large build up.

Good info. I just spoke to the RT people and they said the same. They did alert me to the RTD position relative to the pit wall (it is on the left side).  It should be about 1/2" from the wall. It was closer, so I tweaked it.  Also some question as to whether the Maverick I am using is truly correct.  I have tested in in boiling and ice water and it is on point. In addition was asked to set all the parameters at the factory settings (auger feed, temp adjustments etc). All easy to do with this control.
So I did another test this noon.  Set the control for "ful" (which will allow it to go up over 500).  In 42 minutes it did have a faster rise initially but then slowed down to a crawl and finally stuck around 491.  The Maverick read 525 at center grate position.  Not bad, as ambient temp was 39*. No wind, and under porch roof. Reported same to the tech and she said not bad but she has no problem getting her 700 up over 530 (or such) in the same weather recently. So will adjust pellet feed setting to 75 (standard is 65) to see if that helps, since it seems to me that the pellet feed rate was not adequate to get the temp higher. (if the RTD is Ok)
Note: The pellet feed rate is a setting that allows you to keep from overshooting the low temps that are used for smoking in the summer. Kinda like the P setting on the old digitals. You can set it lower for summer when you don't need as many pellets per minute to maintain low temp  i.e. 170*. Then in winter you will need a higher feed rate for the same effect. It it supposed to be for low temp control, but could slow the feed on the high temp as well.(?maybe?).
I doubt this is the problem.  I am betting on a damaged RTD.  She said if this does not work, then they will send a new RTD. I have stopped the progress on an insulation overcoat at this point as the problem may well be resolved here.  Keep tuned.

Well I was wrong again.  RTD is ok.  Setting the auger rate higher to 75 (not sure what the units mean), resulted in a nice steady startup and steady rise to 500* in 45 min.  It took just less than 10 minutes to get to 225* and 16 minutes to get to 400*.  Nice bell curve up to 500.  Ambient temp was 36*, with 2-3 mph breeze.  Control set to "ful". It would have gone up a bit more but did not want to take the extra time at this point.
As far as I am concerned, this is very good for the current weather--problem solved.
Summary: 
1.  Make sure the probe is not too close or touching the side of the barrel.
2.  As a lower Auger feed rate helps to maintain low temps in the hot summer, so too does a higher than basal rate help get to high temps in the winter.  This adjustment is simple --just remember to change it for the seasons.