Intermittant Coolant Temp Problem

My '81 1.8 GTi Cab is experiencing some random, intermittant coolant problems.

Every now and then the gauge goes into the red, complete with flashing LED and sometimes stays there, and other times returns to normal afetr a few minutes.

Have been keeping an eye on the oil temp whilst this happens and it does rise, but at a slow and steady rate.

Any ideas what could be causing this?  The system does need a flush and replacing with new coolant, but could anything else be wrong?

Do the temp spikes happen when stopped in traffic? - if so, the switch that cuts the fan in is probably tired (it's screwed into the nearside of the radiater body, down low).

If they happen when you are driving normally, take a look at water pump, drive belt, and the thermostat…

Is your coolant VW G11 (or similar)? It contains wetting agents (which aid cooling), rust inhibitors, as well as anti freeze agents. Plain water won't be causing these temp spikes, but won't be doing much good to your engine and water pump innards either…

PS: Are you sure that the engine really is getting hot, and it's not just a faulty reading from a bad connection - take a feel of the top rad hose when the car is hot (but not overheating), and compare that with when it's reading high on the gauge - the difference should be obvious.

on both my gti and gls, I had to change the fan switches they dont seem to last long so I have a spare in my car. I live in Spain, and dont fancy driving in the summer with the heater on.
I didnt see the oil temp. increase when my fan switch failed.
When I took my car for mot(itv) a few weeks ago my fan switch suddenly guit, so I connected both wires to each other and the fan was on permanent.

greetz yal

Yep its the Rad Fan switch! had it on my last one. only about ?6 and easy to replace it a kinda bolt thats it! otherwise short the wires out for those M25 moments.