Well, 88 is the high end of the ideal temperature range. It would be better if you could have a lower temp in the viv as well, to allow your beastie to thermoregulate. I can't offer advice from personal experience - high temperatures aren't something we have too much of a problem with in the UK! Can you set up a fan outside the viv, but near one end? That might take down the ambient temps around that end of the viv and give the heat gradient that you need.