its powered from a lab power supply because it bought it without the original one. also the frequent resets were perhaps caused by the instabiloty of the internal power circuit rather than thr external one

i had similar problems (as well as no tape i/o, colors and restarts after attempting to execute some more advanced commands) before i replaced the tr4 and it was caused by the lack 12v and -12v rails so maybe 12v rail that powers ram was unstable in your machines (due to a bad capacitor or transistor perhaps)

replacing transistors in the speccy is a bit problematic because no one sells ztxes, but i managed to find the tr4(ztx 651) one (but for example tr5(ztx 213) is apparently not manufactured anymore and i suspected that to be broken too so i just bought one with a similar hFE facfor but luckily i didnt have to replace it)

also which issue of the speccy did you have? mine is issue 2