1992 Other Oldsmobile Models Coolant sender / sensor problem

3.8L Olds 98. Intermittant (digital) display of coolant sender problem as well as check engine light plus temp flashes hot but still reads normal. A new coolant sensor has been installed (after computer scan) but it was just as bad the next day. During problem, car is sluggish, runs poorly but can get up to speed. Also speedo and certain other gauges reads 0. Rad coolant is in poor condition which I should change. Mass air, coil packs and module as well as crank sensor have all been replaced. All plugs fire good. There is no indication of when the problem will occur or fix itsself. Thanks in advance for any suggestions.