Help Diagnosing problem 4L60E Tranny

I've searched manually and with the search bar and haven't found anything that addresses this specific issue, only somewhat similar things, so PLEASE dont flame me for posting this if it has been done before:

I have a 1999 Z28 with the 4L60E and 160,000 miles. Recently torturing me and my car in the southern GA heat I noticed that I was getting an intermittent slip in 4th gear. I would promptly shift into 3rd for a bit with no slip and cruise for a bit before putting it back in 4th. Now I'm back in MD and noticed that while I commute to work (usually in the 80mph range) everything works fine while the engine is cold. Shortly after things heat up I start getting the slipping again especially when the load on the engine changes, like from a hill or a sudden depression of the right pedal. As soon as I detect the slip I get off the throttle. Sometimes upon reapplication there will be no slip, sometimes there is. Again I pop it into 3rd and there is no slip.

Fluid and filter were changed about this time last year. T flushed the tranny every other year since 1999, avg about 40k miles btw. flushes. MIL is not on.

Where should I start my diagnostic quest? I am pretty confident in my abilities, but I have never messed with a transmission before...

you should have a 3-4 piston worn out I had the same problem and solve it with a new piston and a 3-4 clutch pack, but you have to remove the tranny first, and Ill suggest you to stop using the tranny or it'll cost you more

you should have a 3-4 piston worn out I had the same problem and solve it with a new piston and a 3-4 clutch pack, but you have to remove the tranny first, and Ill suggest you to stop using the tranny or it'll cost you more

Thanks for the advice. In general, How does this compare cost wise with a total rebuild? I have a guy who told me he can do a rebuild and new Torque Converter complete for $1800.... Considering that the rebuilt tranny I was looking at buying was $1200, this sounds like a deal, especially since a tranny swap books out at like 7-9 hrs.. Is the piston/clutch pack something that is doable for someone who has never disassembled a transmission before (aside from droping the pan changing the filter and doing a clutch on a manual)?

to change the piston you will need some special tools, i suggest you rebuild it complete