lately my truck won't hold onto idle after I start it. Basically it cranks up, RPMs goes up like normal, as the RPMs goes down to where it would stay to idle it just dies. Doesn't matter if the truck is cold or warm. I usually have to hold the gas pedal to keep the engine running for alittle bit until finally it'll hold onto idle. I've checked the MAF sensor, PCV, any loose valcume(sp) lines, a bottle of fuel injector cleaner....and it doesn't happen all the time, maybe once or twice a week. I've even tried getting gas from different vendors(Shell, Chevron, etc.) but that doesn't seem to make a difference either... what could be the problem? Thanks in advance.