What could be the problem with a 94 Chevrolet Suburban C1500?

[Antonio Martinez]

my dad and i were driving on the highway and the traffic slowed to about 40mph and once it picked up to about 60mph the truck started to shake and once it cleared about 65 it "jumped" forward and the heat meter sky rocketed, we checked to see what was wrong but couldnt notice anything wrong it was already night so we probably missed something anyway once we reach the exit of the highway on a red light, the car rpm dropped below 1000 and when my dad accelerated the truck it shaked again and stalled, now then, we took it to this mechanic my dad knows and he said that it could be the timing chain, so he changed it and i think did something with the timing aswell. we also did the usual maintenance routine oil change ,filter etc, and changed the fuel filter, spark plugs. now it seemed to work for a while but a month ago i noticed that the mpg dropped a lot ,the truck hesitates a bit when running at 60 while on overdrive and the rpm is a around 200-400 when on idle, im surprised it doesnt stall. so im wondering what else could be wrong, yeah sry about the long story.

 

[Magnum44]

Just shooting from the hip Antonio but possibly the Oxygen Sensor. Unfortunately there could be multiple issues but I would start there. Even something as simple as a dirty fuel filter can cause havic with the fuel injected rigs.