My 99 Jeep Grand Cherokee is stalling out. If I'm standing still it just dies, but if I'm moving I can pump the gas and sometimes it will come back alive. Most times I have to slide off the road and continue to try to start the engine. This will take anywhere from 2-5 minutes, often longer. This started about a year ago happening 10 times in 10 months, and has escalated to every time I run the jeep for the last two weeks. What could be wrong?