Non pug car problem

[Moonstone2.0]

New problem with my parents 2000 Golf V5.

Last few weeks there have been strange things happening. Heaters being random. Cold on hot, hot on cold and sometimes just not working. Then a juddering from the engine. Really bad under load from time to time. Sometimes the car is fine but sometimes it feels like it has square wheels. Really underpowered when the juddering is there. Could be running on 4 cylinders and knocking about. Then the built in navi goes crazy. Restarts, cant find places and just stops.

My mum told me she had this problem without the juddering (just electrical faults) two years ago and one day it just totally cut out in the middle of the road completely dead. AA took her to kwikfit (how dare they!) and they apparently (who knows) installed a new alternator for £700 and its been fine since the other week.

Engine management light has been on for 5 years and no garage has ever been able to clear it so cant tell if that would have come on lately.

Drove to work today without any extra electrical on and the car ran ok.

Any ideas? Got my wedding in 12 days and cant afford to fix it if it dies...great timing!