My 99 2.8 a4 is fine when starting it up but has started to seemingly choke when I'm at speed and the engine light starts blinking. Once it starts doing this, it continues at any speed or when idling. It shakes violently, lurches and sounds as if it's not getting enough fuel. I did the spark plugs a year ago and nothing is visibly leaking so I'm not sure what's wrong

 

Sounds like it misfiring. When the check engine light comes on and stays on it usually means something needs to be looked at but when the engine lights starts blinking while your driving that means something is really wrong and you probably shouldnt be driving it until its fixed.. with my Audis, fords, and GMs 90% percent of the time it comes back to misfires of some kind. My Mustang was doing this same thing recently and the engine light would start to blink and it ran very ruff when driving at idle it shook the whole car and with the hood up you could see the engine vibrating and shaking all around. When i scanned it the computer said misfire in #1 cylinder and bank 1 running lean and so on. Turns out the harness going to the #1 fuel injector came loose somehow and once that was clipped back in and i cleared the codes ran like a dream again. You need to scan it and figure it out before it causes a big problem when the engine light blinks light that it means something is very wrong and it would be in your best interest to fix it right away rather then continuing to drive it in that condition.