Startup woes - vertical coloured lines and PRAM

During my battery problems (see previous post), I had trouble starting up my Mac, even when the battery was in and had charge. I'd get one of two problems: either a grey screen when powering on and then... nothing or vertical coloured lines as below:

An extended Google search found the solution on the Apple site - the PRAM needed resetting.

If that doesn't work, try resetting your computer's PRAM (parameter random access memory), then repeating the X key steps above afterwards.

1. Restart your computer and immediately press and hold the Command, Option, P, and R keys simultaneously before the gray screen appears.
2. Continue holding these four keys until you hear the startup sound for the second time.
3. Release the keys to allow your computer to start up normally.

Unfortunately, the original machine I received needed resetting so often that I'd always start up while holding the above keys.

Luckily, the replacement machine doesn't have this problem.