Ten Metre Odd stuff

I have recently been beaconing on Ten metres and have been interested to see on the MAP how propagation develops over a 24 hour period and how seasonal variations appear to affect paths. But something I have noticed which may be an indication of a problem at my station is this.......
After a pre sunset session of good decodes......
If I leave the rig running over night as you may expect during darkness I get no decodes or signals as far as I can tell. But as dawn arrives I can see lots of signals some at very good strength, but non of them decode. I may wait several hours before decodes occur. I have thought this maybe a timing issue and have this morning changed the time zone so I now use UTC on the PC, to ensure that there are no obscure timing problems. It now remains to see if this will solve the problem. Presently I have "Dimension4" synching time every 30 mins.
I don't see that VKs with their sunrise 2 or more hours later than me are having this problem..... during one of these periods of no decode I have gone down to 20 metres and have had no problems.....so maybe it is a frequency stability issue after all.