the commodore monitors work fine but do need the link on the ula shorting as they have a high impedance input on that composite input.
from the scope trace it does look like your are getting 2 h-syncs in very short space of time, which is very odd indeed and the onlt thing that can cause that is ~M1 and ~IORQ both going low at the same time and resetting the internal counter. i guess it could be a bad connection on either of these signals. Or but this is wildly speculative some Z80's were known to have a problem with weak M1 signals BUT since as a good M1 signal is requiered for the standard ULA to operate correctly this would seem to be a stupid suggestion but if the CPU's are socketed it could be worth trying a known good CPU.
sorry it's took so long for me to get on this thread have had a busy crazy weekend.
@Mark yes got a 26Mhz osc module on board... the really 2011 ULA i did had a sort of colpitts osc and was dodgy to say the least... so just went for this will work every-time approach.
I have had it running for several hours now with no problems. I really didn't do anything other than leave it on for quite a while and the artifacts stopped. I have let it cool down and start again with no issues. Really weird but I am glad it is working now.