tranny or engine? ('95 Suburban)

I've got a '95 1/2T Chev Suburban 4WD with 147K miles. I'm assuming the auto trans is a 4L60-E.

I've never had any issues with the burb at all, and took off on a 400 mile trip a week ago. It ran fine, but about 100 miles into the trip, I had a slight incline on the freeway, and went from 70 mph down to 50-55 mph. Then the service engine lite came on. As I got over the incline back on level or slight downhill grade, it immediately picked back up to 70 and engine lite went out. This occurred again a couple times for the next 10 miles, and I pulled over and popped the hood. Everything looked fine, so I pulled trans dipstick - it wasn't reading, and a little bit of black stuff on it. Of course I'm in the middle of MT and the middle of nowhere - so I turned around and drove 10 miles back to where I could get some ATF. Dumped in two quarts and decided to try again - had the same issue, so I said the heck with it and took it home. It shifts fine, and runs in OD no problem, but still had no power up any sort of an incline. The fluid level is now normal when idling in park with the engine warm. Color is not the burnt brown - still deeper pink, I guess (i'm kinda color blind)....

Somebody asked me a few more details - I told them it shifts fine and at the correct points from first thru OD - I said there's plenty of power in first and second, but it seems to lose power in drive and OD. He said if tranny is the problem, I should see engine revving up as tranny slips - which it doesn't do. It just seems to lack power in those two gears. He said there are two sensors in the distributor - one's an idle sensor - I forget the other, but he suggested I replace the non-idle sensor - the vehicle has almost 150K miles - the tranny fluid isn't the burnt brown look...but he thinks it's more of an engine issue since it isn't slipping. Does that sound possible?

First scan it for what the codes say, second try checking the fuel pressure particularly under load/speed. Post back these test results.