Why is the glow plug light coming on sometimes?

typical dealer!!! talking mince!!!
If you had a broken w/f key your engine would by now, be scrap. Have your alternator checked, rev counter gets its signal from it. Also check the battery terminals/connections also where the earth lead attaches to the body. I had a customer I sent to Renault dealer to have a new camshaft fitted, £6000 later it had a new engine. Heater lights do come and go on these, usually down to poor earthing somewhere. Also try and change the fuel filter for the initial poor running.

JBM
February 2009