My 2000 jimmy (vin w 4x4) has developed some strange issues over the last few months. Truck seems to run great sixty percent of the time, but sometimes it is completley gutless, terrible on fuel just a general pos. When acting up ill be driving and take my foot off the gas and the revs drop to four hundred or so, then bounce right back and drop again. It will do this until im back on the throttle. Anytime you give a little more throttle to gain speed, it downshifts right away. Wont go up a slight incline without jumping to three thousand rpm. When it runs normally none of these problems occur. Doesnt seem to be affected by moisture or temperature. Im stumped and could use some help. Thanks in advance.

I should add the truck also wont coast when its acting up. Almost like its standard and I dropped a gear. Otherwise it will roll for a good long time

 

Anyone? Someone must have an idea at least?