My daughter owns a 2000 VWB with a 2.0 liter engine with 175,000 miles. Recently, the engine started chugging while going driving down the road. It runs fine, then will stop like it is out of gas, then start running just a quickly. I assumed it was an intermittent fuel pump, but after checking, the pump runs when the key is turned on. I have replaced the fuel filter, used fuel additive for possible bad gas, still with the same results. Help!