'98 A8 Quattro - Whole car shakes when pushing accelerator above 3rd gear
 

The other day, my 1998 Audi A8 Quattro started shaking after I started it. It was shaking (rough) as I was idling. However, when I began to accelerate up past 3rd gear in automatic mode, the whole car would shake and would become loud.
If I shifted in tiptronic mode, I could avoid the heavy shaking by keeping it in 1st, 2nd or 3rd gear. The next day, I drove it to another city to take it to the repair shop. After approximately 20 miles, I stopped and turned the vehicle off to add gas. I filled up the tank with premium gas, as usual. When I began to drive again on the highway, no problems and no shaking or loss of power.
Could it have been bad gas? My tech recommended changing the timing belt as it has 145,000 miles on it. I bought it with 106,000 a few years ago and have had no problems with it.
Has anyone else experienced this problem?
What did you do to fix it?
Thanks.
Hans

The engine is having a running issue and the first thing your tech says is to replace your timing belt!? Sounds like a gravy collector to me. He would hit you with a $2,000 repair bill and once he's in there he would call you up to upsell what it actually needs. Find a different technician that won't rip you off and can actually DIAGNOSE the problem.

If a new tank of gas made the car run normal (and continues to do so), you've solved that problem. If it didn't, it read like you were describing an ignition problem that would NOT go away with new gas. A single failed coil or spark plug would act like that. I lost a single coil a while back, and at lower rpms it would shake the car apart...downshifting to high rpms would smooth it out considerably...just masking the problem, though. A consistent complete misfire on one or more cylinders triggers a flashing check engine light and a code reader will tell you which cylinder (mine was #8). Easy diagnosis, easy fix.

As for your timing belt...nothing to do with what you described...no way.
But, if you have the original T-belt belt at 145K miles, yes it's past time to change it. I'd have another mechanic actually check it...it WILL be obvious, if it's the original.

Plus I'd want to look at the spark plugs, soon...even if you had a tank of "bad" gas, old plugs would be much more likely to misfire...and it sounds like you were getting ignition misfiring for what ever reason. Starved for fuel shouldn't run better at high rpms...it should be worse. And a trans problem wouldn't get better with new gas.