Yeah I know, I did stick my neck out. But it was for a worthy cause!!!
Only thing that makes sense about the "something electrical causing it to slip" is perhaps he didn't tell the dealer his trans is a manual?
And, so far so good with your trans 6.5, no slipping gears...yet?

 

Lol, well, after 60,000 miles, I am finally letting myself hope that the GM guy was wrong.....

 

mine did that, only in first and second and shifted really hard sometimes and not so much other times, mine is an auto tho... turned out the torque converter was worn out... so i dropped a edge racing converter in there and BAM, car came alive and no problems in that area since...

 

I hope now that GM is a "new" company they will drop the old training books they had the techs read. GM training said that we were not supposed to use anything other than GM parts (not even the same exact part from the local AP store), Cant use anything other than the GM fluids, and we MUST tell the customers that they HAVE to use the GM brand stuff or their cars will... not be covered under warranty, will cause damage... etc, you get the idea. I was never happy about having to follow these rules. When I questioned this in my training I was told that I was to follow these rules or I would be fired. Most of GM's stuff is pretty good but there are stuffs out there just as good and will NOT void your warranty. Be careful.

OH and if your car is a 5sp, and you are having issues when you are on it hard and not when you are cruising, I have to go with the others on that limb here, (it is getting pretty heavy out here) and say you need a new clutch. Soon and I do mean very soon when I say soon, you will be slipping at all times. Slipping is generating heat which could and most of the time will cause your flywheel to crack and get heat spots on it. Yes they sound bad and in reality will kill the new clutch really fast.

Massey

 

Massey is right about the new clutch. You'll need to replace it very soon. Another way to test the clutch is to shift it in to reverse and if it doesnt go in smoothly then you clutch is worn out. I know that it the clutch as my car has just started doing the same thing, engine rmp will rise but it feels like the car lost the forward momentom. A new clutch is any where from $150-$400 depending on if its stock or a performance clutch. instillation is the biggest hit in cost unless you know how to do it yourself.