You should be able to use it, it's not going to melt down. Though frequently running at 90c isn't exactly good for the cpu either. It depends on how long it's run like that, for a few days or a week not really an issue, for another several months it could degrade the cpu somewhat. If anything I'd check it for dust and try dusting it out before reapplying thermal paste, the thermal paste may be fine. If dusting it doesn't have much effect then I'd consider reapplying the thermal paste and reseating the cooler. If there still isn't much change then I'd look at the cooler to make sure the pump is running like it should be or consider replacing the cooler.
Replacing the cooler may be an expensive fix to simple dust build up, replacing the...