I know this thread is a little old, but it is similar to my prob on a 93 aerostar 4.0. Guage does the same. Fixing to do water pump/thermostat/sensor/check connections. Indication is that pump or thermostat is not functioning/opening. Will first check new Thermostat by hanging in a pot of water on stove w/ temp guage. I see several posts where a thermostat change does not seem to help. I have been bitten by that one several times, until I found out that even new thermostats can be bad. I always check new ones now. No apparent water leaks anywhere, including the tailpipe.