Look at: https://68kmla.org/forums/index.php?/topic/23844-mac-plus-temporary-jailbar-screen/
Sounds to me like your Mac is not reaching its required voltage. Its either a part on the analog board or the caps going bad on the logic board, which is rare for a 128K. I forget which pins it is, maybe Uniserver or TechKight can post it, but you can check the voltage on the floppy port on the back of the Mac. If it registers less than 5.0 volts, you could get to the analog board and turn the potentiometer/variable resistor for 5 volts. Turn it a bit while it is off, turn on the machine and give it a while to come up and measure again. Turn off and adjust again as needed. It is better to be a bit over 5 volts, like 5.1 volts on your digital multimeter or the needle a tiny bit past the 5 volts mark on the analog meter. I use both.
If you still have this problem after adjusting, then its time to look at the analog board or the caps on the logic board.
Sounds to me like your Mac is not reaching its required voltage. Its either a part on the analog board or the caps going bad on the logic board, which is rare for a 128K. I forget which pins it is, maybe Uniserver or TechKight can post it, but you can check the voltage on the floppy port on the back of the Mac. If it registers less than 5.0 volts, you could get to the analog board and turn the potentiometer/variable resistor for 5 volts. Turn it a bit while it is off, turn on the machine and give it a while to come up and measure again. Turn off and adjust again as needed. It is better to be a bit over 5 volts, like 5.1 volts on your digital multimeter or the needle a tiny bit past the 5 volts mark on the analog meter. I use both.
If you still have this problem after adjusting, then its time to look at the analog board or the caps on the logic board.