Ah yes. I think we've all wondered about this at some point in our lives. Why some men (and indeed women too) run hot and cold will depend on the nature of the relationship you have with a particular guy. Since we're in RedHotPie Land, I am going to assume that you met that guy for a easy-going, casual relationship with view to something more serious happening, if it happens?
Going by the profiles I read on RHP, that seems to be common thought for a lot of men and women seeking fun online. Why some such guys who seem otherwise open to 'whatever' start blowing hot and cold seems to be due to one of two reasons I can think of; either the 'something more serious' gets them running scared when it eventuates, or they find that oddly enough, having one FWB relationship actually makes it hard for them to pursue and enjoy others.
If a guy has found himself getting emotionally involved with a playmate, or vice versa, he might think it's wise to put some distance between each other, at least until the urge for hot sex arises again. If it's what a guy needs to do to try and maintain emotional detachment in an otherwise highly fulfilling sexual relationship, then you might have to put up with it or cut him loose. If of course you'd like to entertain the idea of developing 'something more serious', it's time for a talk. How ever it turns out, at least you can stop adjusting your thermostat!
Other guys may have zero concerns about their own emotional state but still feel awkward or even guilty about having a FWB *and* still wanting more. It can be weird for a guy to have good, fuzzy feelings about a long term sex friend when later that night he's planning to meet a potential new playmate. In his defence, he just doesn't want to offend anyone or spoil the apple cart. Running cold in this scenario may just mean that he doesn't want to feel bad about wanting more than one FWB arrangement. If it means alienating one friend just a little in order to feel OK about pursuing more sex, then he'll hold back on the friendly phone calls and post-sex cuddles until he sorts his proverbials out.
If it doesn't bother you one iota that your friend wants other NSA encounters then you need to tell the guy that he can run as hot as he wants with you without needing to feel guilty about wanting to run hot with other people. Of course, if it does bother you then you need to set him straight or set him free.