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I appreciate you have tried different set top boxes, have you tried a different HDMI cable, as there is a sync process as part of HDMI which should stabilise the image and this can stop working with faulty cable, and may be causing this issue.
Secondly, where are you switching the aspect ratio on the Set Top Box or the TV itself?
Is there a OSD within the set top box to do the same, I am wondering if the format the boxes are transmitting on is 14:9 or NTSC rather than PAL, which may cause this type of issue.
The other consideration, is it may be the digital encounter within the TV, if your confident the set top boxes are set up correct (and working on other TV's) then the only other thing I can suggest is a factory reset of the TV, failing that its looking like a faulty main board, which would probably be uneconomical to repair.