Well a friend of mine purchased a 99 deville 169,000 miles on it, it's not losing coolant or slimey tail pipes, checked the purge line flows steady but I noticed that the car will run 196 -199 for about 15 minutes driving, then the temp starts climbing to 222 -226 and goes back down to 213 and starts climbing again. I noticed when he got on the freeway the temp was 192 and he gunned it up the ramp and it and it climbed to 196 and stayed around there. Well we left the car off about maybe 5 hours and I took off the radiator cap and had him idle the car, and stood there for about 15-20 minutes and it didn't overflow until it hit about 217 the coolant started to rise slowly so I put the cap back on. Is this normal? it should boil over within a couple of minutes right? maybe a sticky thermostat or something?