91 vw golf will not stay running after 15 minutes

Get the codes read, that's the only way you're going to find out what is going on with the vehicle. You don't have to have the dealer do it, all repair shops have code readers, along with most auto parts stores and some of the oil change places can do it also. With as computerized as the cars are now, when the CEL comes on you what to read the codes.

My immobiliser keeps flashing until I put the key in the ignition and then it stops. Normally changing key batteries has a period that if the battery is changed inside that time (say 5 minutes) then the key program will not loose memory Run fault codes to see if there is a problem in the electronics. Otherwise you may have to take the car to the dealer to have the key reprogrammed to the cpu in the car

Most likely you have some clogged oil passages. You can try a cleaning additive but I prefer to substitute 1 quart of oil for 1 quart of transmission fluid during an oil change. Run the motor (in the driveway) for about 10 minutes, drain it and refill it with clean oil that the manufacturer recommends.

If that doesn't work, you may need to replace the engine.

It is very possible the culprit is the radiator cap.

a car of that year should have an ecu controlling fuel air mix etc. this normally controls the timing.
To reset the ecu on my '95 VW Golf disconnect the battery (negative / black terminal) for 15-30 minutes depending on who you ask. So I do 30 minutes.
If it has central locking make sure you don't leave the keys in the car when reconnect it. swich on the ignition without starting the engine so the dash lights up with the oil light etc. for 30 seconds then off for 10 seconds. then start your car. It will relearn so it will run a bit rough for a while but should settle down. take it for a spin on different types of road ie. slow and fast.

Could be oil warning signal, check oil level, adjust to correct level.

Measure your oil at normal operating temp, without the engine running of course. This will give you a more accurate reading once the oil has circulated in the system.

The system works like this:

• With the ignition on, the oil pressure indicator light on the dash should come on and flash. Once the engine is started, the light should go out and stay out as long as the engine is running.
• If the oil pressure drops below approximately 0.3 bar or 5 psi when the engine is running below 2000 rpm, the light will flash.
• If the oil pressure drops below approximately 1.8 bar or 27 psi when the engine is running above 2000 rpm for more than 3 seconds, the indicator light will flash and the buzzer will sound.

If either this two warnings occur:

• The engine should be switched off immediately and the oil level should be checked.
  

try changing the oil, paying great attention to manufacturer's recommendations. Could be oil is way too thick and is sticking things that are supposed to slide, until it warms and thins.

lights need turned off by a scan tool, some cars you can just unhook the neg batt terminal for 10 min and y0ur good to go.......

yes it could be the cause of the problem