PID issues?? Folger Prusa i3 cannot maintain 185C. Can maintain 186C.

interesting issue with my Folger tech Prusa i3 lastnight. I was printing a temp cal cube and everything was going fine until the target temp dropped below 186C. The temp was not staying at 185 and was cycling between 175C and 184C while the heater power cycled between full on and full off. When setting the target temp for 186C the heater cycles around 50% and the temp stays rock solid +/- 1C. I've looked at the PID settings and I am using the lasted Marlin firmware settings for the Ultimaker. I'm am hesitant to make changes as it seems to maintain temps at 186 or above perfectly fine.
Why would a target difference of 1 degree (186C works when 185C doesn't) make such a big difference?

Its the Marlin 1.02? It's full of bugs related to temperature and abl. I was testing it for a while yesterday and after about 4 hours of problems whent back to 1.0

Yup I am on 1.02. I checked out the github and didn't see any reported issues similar to mine so I submitted a new one. I have experienced a few of the other issues noted as open. Now I'm glad I haven't made it to set up abl yet. It would have been a nightmare trying to set it up with buggy code.

I'll try downgrade to 1.00, I'll report back in once complete.

Turns out I had a faulty thermistor on my hotend. I was able to get it to print well enough to mount my e3d hotened wade setup and now the temp is rock solid wherever I set it. thanks for the help everyone.

Glad you solved your problem, another time if you cant maintain your temperature you could try to do i pid autotuning, it worked for me