I just recently purchased this car and it has 190,200 miles give or take a few on it. It needed a tune up bad so I did a tune up, plugs, dist. cap, rotor, cold air intake. Also replaced left front steering knuckle, and coolant reservoir. The car runs great when you first fire it up and its still cold but once it reaches normal running temperature and you come to a stop and begin to take off again it cuts out and dies when throttle is pressed. If I keep the rpms at around 2,000 when I come to a stop it will not die when taking off again anything lower it stalls out. First thought was fuel problem well it has a new fuel filter and I've ran two bottles of cleaner with two tanks of premium gas and its still the same. Plugs are gapped fine like i said when its cold it runs with no problems. Well this morning I left in the car to make a 10-12 mile trip or so and probably 3-4 miles into the trip I look at the temperature gauge and its sitting just under 230 degrees but rising. It reached 235 degrees before I found a place to pull over due to weather conditions. Well first thought was it needed coolant well it was almost completely full when i checked the level. The concern i have is the coolant in reservoir is boiling and once you break the pressure its pouring out the top but just for a second then it stops but once it stops you can watch it like suck inside and then back out to the top of the reservoir like pulsing. My first thought is thermostat maybe stuck because the car never overheated enough to cause any damage. Well now I'm thinking it could be the coolant temp. sensor that may need to be replaced because the temperature gauge will move from Normal to 230 degrees and back again it moves slowly but it does move. Also during this process the heater will start blowing cold for a few minutes then start blowing warm again in a few minutes. If someone can please shed some light on this for me that would be greatly appreciated, I'm new to VW's but I'm eager to learn so please don't bash to hard haha. FYI there is no check engine codes or anything else being thrown. Sorry for the long thread but I wanted to explain in detail what is going on so this can be resolved in the easiest way possible. Thanks!!
All of your symptoms point to flow issues. Don't waste your money on a coolant temp sensor. You'll likely notice, if you pay attention, that when the heater blows cold is when the temp gauge starts to climb. That's due to lack of circulation. The water in the engine stays in the engine and gets hotter.
I would start with replacing the thermostat. A stuck thermostat WILL damage your engine if you let it get hot enough! I think it isn't stuck, it's getting sticky and isn't opening when it ought. It will eventually stick for good.
The reservoir thing is happening because the water isn't moving, so it gets really hot and boils out...then the 'stat opens and you see the "vacuum" thing happening because cooler water is rushing through the engine.