Yeah, I know this is an old thread, but here's some advice I recently learned.My '88 has had a slight leak for a while now,. It leaves a few drips on the garage floor over night and requires the reservoir to be filled ever couple months. It's leaking at the rear of the intake and I've been putting it off, but am fixing it this weekend. Anyway, I recently took a trip to the SF bay area with the car and while it was hot out, the car ran fine. But after stopping for dinner, the car pegged the temp gauge back on the freeway. I pulled off at the next exit, found a station and shot water on the radiator to cool it down (which it did not), then wrapped a towel I keep in the hatch area to dry the car off around the cap and removed it, expecting a spray. But nothing happened. No pressure at all, and the radiator was full and cool. Not only that, but once back on the freeway, the temp gauge stayed on the "N" part of "Normal" for the next hour until we got to our motel. The next morning, the thing started right up, and the temp gauge was right in the middle as if nothing happened. Once we got home a couple days later, I replaced the thermostat and radiator cap and have not had one problem since. In fact the car runs cooler, warms up quicker and the heater works better since then. The downside is the coolant leak in the intake got worse, I suspect the old original cap hasn't been building pressure for quite some time. Anyway, Erratic temps are sometimes a result of a bad thermostat, it's cheap and easily replaced, so why not try it?