A bunch of Silverado ????'s

My 2000 Silverado (http://www.automotiveforums.com/vbulletin/../#) 1500 5.3 4x4 will skip or shudder when I'm pulling a trailer or just going through hilly country. When towing I use the tow/haul feature in drive not o\d and usually set the cruise at 70 mph. If I encounter a hill where my speed drops back to around 68mph the skip/shudder starts. When the transmission drops to a lower gear the skip/shudder quits or is not noticeable. It does it in the mornings on level roads in o\d but doesnt do it drive. The other day when i was at a light and went to go it felt like the tranny slipped then it banged into gear. I have been told it may be a senor or something for the shifting problem.
Also when climbing a tiny hill on cruise control my speed drops 1 mph it will then kick down to passing gear and sometimes it wont. I thought it was around 5mph it should downshift but not to passing gear. Sometimes I almost have to put it to the floor to get it to downshift for some power to climb the hill. I am towing a small 10ft open trailer not a big thing at all. I was told to check the cables for slack and they are good in the regard. Thanks for any help you may have.

also forgot to put that tranny fuid is full and it had a complete tranny sevice at 100,000.

When was the last time it seen new plugs wires cap rotor??

99-up v8 Silverados do not have a cap or rotor.

Woops!!! missed that, yes i know..lol

I had the same problem with my 99 with a 5.3. I had the injectors cleaned and the problem went away.

Well, I think it was the EGR valve that may have been causing the problem. My check engine light came on today and it gave the code for a bad EGR. Replaced it and now it climbs hill and all without kicking down on cruise. Must have been loosing power when it was starting to go bad. Now if i can get the shifting issue resolved.