I've been experiencing the exact same thing you have.
On the way in to work this morning, I noticed the EVIC displaying 21.9 L/100. I reset the EVIC. Display was reading 75.x L/100, then came down to a more reasonable 17.x L/100. Then it came down to like 11.x L/100 and slowly crept up to between 14.x L/100 and 17.x L/100.
Sitting idle at a stop light, I noticed the EVIC creep up. First it read 17.8 L/100, then a few seconds later it read 17.9 L/100 and then 18.1 L/100, etc. I realize the L/100 should change as I idle, but that quickly?
How can I go from 11.x L/100 to 17.x L/100 in under 1 km? On a 121 Litre tank I am reading 591 km (DTE) at full. In the summer my EVIC was reading well over 750 km (DTE). What the heck? The last tank fill read 520 km (DTE)!!!!!! That's some pretty lousy gas mileage.
When the truck was new the EVIC would show between 20.x and 21.x L/100. Then I went on a road-trip to B.C. and was averaging 11.x and 12.x on the highway. Pretty impressive, I thought. City driving was around 14.x and 15.x in B.C.
Now, at just under 10,000 kms, I am averaging around 19 L/100 in the city.
Very frustrating.
But, seriously, getting 520 kms city driving from 121 Litres seems a bit on the PIG side.
Anybody else seeing this?
Suggestions?