Chrysler Repair: 2000 chrysler overheating irratically, head gasket leak, postal mail

Question
my car overheats unexpectedly. for aexample i drove for 3 hours in traffic jams mostly during noon and nothing happened. but when i tried to go dinner, it overheated just in couple of minutes. i already changed timing belt, water pump, radiator cap,coolant but nothing changed. when itt over heats coolant boils in the overflow bottle and steam comes out. there is nothing wrong about the exhoust (i mean no water in it)and radiotor fan works in both high and low levels. sometimes it over heats even driving at 70 miles.and one more thing: when the heat level passes the middle of the thermometer gauge, it makes jumpes like 1/8 of the gauge. also when the tempature drops (sometimes it drops just while working)it drops suddenly rather than graduaaal. i really need help about it cause my mechqnic has nno idea about it.

Answer
Hi Alper,
You didn't say which engine you have so let me know that if you do a follow-up question. Here are some observations and suggestions:
While it is good there is no water in the exhaust, you might want to see if you have a cloudiness in the oil which can also be contaminated by water, in the case of a head gasket leak.
You may have a problem with air trapped in the thermostat housing or other area of the system. That air can cause the thermostat to fail to open promptly if it is in the thermostat housing and conversely to open promptly when the air is cleared. That would be a cause of rapid rise and rapid fall of the gauge. There is a special procedure you need to follow to refill the system to avoid that, so if your recently drained and refilled the system that may be the cause. I can xerox and postal mail the procedure for that if you have the 3.5L engine.
Do the fans come on when the gauge reads hot? I would stop at the time when the gauge is moving up quickly to check that. If not, that fan control circuit is the problem.
On the waterpump: the 3.5L pump is driven by the timing belt so if you don't need a timing belt change out it would be wise to only get into that issue when you have tried the other approaches first. It is a fairly big/expensive job to change the pump, so that is why most people recommend doing that only when the timing belt is showing sign of wear out and then you change the pump regardless of its condition.
I won't be available for 5 days after today, so see what you can conclude about some of these ideas.
Roland