2000 GMC Jimmy

Engine Performance problem
2000 GMC Jimmy 6 cyl Four Wheel Drive Automatic

I have a 2000 gmc jimmy that has been acting up as of late. When you drive it for the first time in the day it seems to hesitate and feels like it is running out of fuel. When I step on gas it has a hesitaation and then it kicks in it runs fine on highway or for the rest of the drive but when it sits for about an hour or so it does it again I have had it put on diagnostic machine and it was found that the tps had to be replacedbut did not make a difference.I replaced map sensor still nothing ehen it was diagnosed they told me my fuel pump was working fine also which I thought might have been another cause I have noticed a white yellowish milky substance in the the tube that comes from the passenger valve cover that goes to the air intake plenum is this my problem and what would cause this to happen the truck only has 157000km plz help. Oh yeah also did afuel pressure test when it started to act up pressure styesd the same it feels like it wants to stall when this happens also