I have a logic 30 boiler and the water keeps running cold when the shower is on. It happens frequently but not all the time

SOURCE: Gainsborough Shower Running Cold

It sounds like you have a faulty thermostat or broken heater element.

SOURCE: Combi Boiler - Running warm & then cold!? HELP!

mixing valve must be replaced

SOURCE: Shower goes hot and cold

If you are having trouble only at the one fixture (shower), and no where else, the problem is most likely in the shower valve...which either needs new cartridges, or a complete replacement.
hope this helps

SOURCE: WATER RUNS COLD

ITsounds like you still have air in your lines try bleeding lines out some times it takes a while .or your getting air in lines some where try looking at the boiler at fittings .

SOURCE: mixer shower connected to combi boiler running hot and cold

Mixing valves usually have a diaphragm inside that keeps the hot and cold separate. If the diaphragm is old it may be allowing the hot and cold to mix improperly at the back of the valve. This is similar to a cross connection (a hot water line tied into a cold line) in your plumbing. Cross connections interfere with the performance of Combination Boilers and Tankless Water heaters due to their having no water storage or a very small buffer tank. The brand of the valve will help as each one is different in their mechanical design. If this is the problem you or a licensed plumber can replace the cartridge in the valve. Refer to your valve's owner's manual for part numbers.