Gets hot and then cold at random

I have had my heater core and blower motor fixed in last year or two and now it will get hot and then cold at random. I feel like I had it coming out good one time and never shut the off switch just using the fan speed to turn it on or real low and it would stay on for the most part. Now it is not staying hot again. Sometimes I could shut the truck off and turn it back on and get it to work that way. Might take a few times. I have felt the hoses and sometimes the one hose will be warm and the other hot. Other times the one will just be warm. Does it just may be need a flush? Any information will be helpful. Thank you!