W210 wont start on steep hills

[in&out]

When the car (E240 estate yr2000) is on a steep hill pointing uphill or down hill I have a problem starting, starter turns over but its as if no petrol is getting through as the engine will not fire up. If I keep turning the ignition key it will eventially start after about 10 attempts. When the car is on the flat it starts first time every time.

Any ideas what the problem could be, could it be the maf?

Thanks in advance for any help.