Tow /'s EGR replacement? Fuel Gage malfunctioning

[Quincelady]

Hi,
1)A mechanic told me I need to replace the EGR valve. Car stalls when first started. Within a minute of reving the motor..no problem. Do you know if their is a gas additive I can use to clean the exisitng valve she has 73,000 miles and is 1996 C280.
2)My gas gauge shows the exact amount at times and then at times it shoots down to zero. When I start it up sometimes it shows the amount and sometimes it is at zero.While I am driving, at times, it goes back to the amount that is in the tank? She ran out of gas at one point ! Hmmm... any help is always appreciated.
Thank You,
Qincelady