zx spectrum 48k interference during first two minutes of warm up

i have a issue 3B 48k that i am looking at during the first two minutes (approx.) of warm up there is interference on the screen (tried to take a pic but it didn't come out, it looks like a bad video cable or power supply noise etc) then suddenly its fine and will only come back once the specie is left to go cold... the only chip in socket is ULA which i have tried swopping with no difference .... it is currently running a composite mod (via 100 uf capacitor) as the RF didn't find a signal ..i have not yet replaced the caps.. voltages are all ok, i stuck in the retorleum smart card and it reports everything being ok .. any suggestions would be welcome ?? i was thinking maybe a lower ram chip was on its last legs ?? i will prob do the caps in a bit
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