I have problems with running programs or characters changing after the

[Blacklord]

This could be a bad DRAM or even part of the refresh logic. But most likely it is a failing power supply. The electrolytic capacitors in these units are the first part to fail. You can check +5V at the power on LED connector for lo profile units. If it is below 4.9 volts or more than 5.1 volts you need to service the power supply. But first try a simple adjustment, thre is a small screw on one side of the power supply. This is R27, on the lo-profile power supply, and is used to trim the +5V feed. With a digital multimeter (DMM) try SLOWLY adjusting this with a non-conductive tool while checking Vcc and ground on one of the EPROM chips. Adjust to +5.05V as indicated in the service literature of the time. If that does not work you'll need to remove the supply for service. Note: these are switching power supplies, do not test them disconnected from the motherboard without a load or you can permanently damage them. Also these supplies work with HIGH VOLTAGE. DO NOT WORK ON THESE UNLESS YOU KNOW WHAT YOU ARE DOING - YOU CAN GET KILLED OR SERIOUSLY INJURED! In my experience replacing ALL of the original electrolytic capacitors followed by testing/replacing switching transistors makes for a lasting repair. The Motorola MJE 8501 switching transistor is the most likely to have failed if it is a lo-profile power supply. But it is ULTRA rare, even in the late 80s. Go looking for the MJE 8503, STI8501 or BU508A instead. It will service fine as long as you get it in the same TO-220AB packaging. If you have a high-profile machine then the part is a Motorola BUV48 transistor in TO218 packaging and a replacement can be the NTE2311.