does the hot water stop at any other fixture during this time that the shower gets cold? If it does we need to do some things to check your water heater, if it doesn't and it is just this one fixture, then we need to check it out. So some questions, any idea as to the brand or model the shower valve is? Single handle, Two, or Three Handle? About how old is it? If you try to increase the hot water when it is running cold, does it get hot or just stay cold?