i have an 02 f150 bifuel with 84K that i have put 20k on. i have replaced the fuel pressure regulator, filter, coils and plugs a while back, around 10k miles ago. the last two days it has had an issue starting. it almost sounds like a weak battery when it tries to start but when it does take the truck shakes when idling. as soon as i start to drive and it warms up the shaking is gone, any ideas? bad tank of cng?