Clutch Slipping in 4th gear only.

I have a 98 KX250 that slips in 4th gear when the bike is on the pipe. I can cruise around in 4th gear without problems. It doesn't slip in any other gears.
The problem started when I noticed the clutch slipping at the track 1 day. The rubber fill plug broke and was leaking oil. I installed a new White Bros. clutch kit and it still slipped. The oil was filthy after only 5 laps. I took the clutch apart, sanded the fibers on a glass plate, filed the clutch basket smooth(it wasn't that bad), installed kx500 springs and fresh Maxima 80 wt oil and it still slipped.
So I took it apart again. This time I had the clutch hub and pressure plate faces glass beaded, put in new OEM fiber and steel plates, the 500 springs and Bel Ray 80 wt oil. The clutch didn't slip after 6 laps. I pulled in to check the oil and adjustment. I went back out and the clutch started slipping after about 5 more laps.
I took it apart again, very carefully inspected and measured everything and couldn't find anything obvious. Actually I've taken it apart and inspected it about 10 times over the course of 3 months.
I was an auto mechanic for 8 years so I have pretty good mechanical ability and I have the service manual. This has me completely stumped. I called Hinson and they said it could be the basket but I dont want to spend another $200 and not fix it. The basket is still within spec after mild filing. Is there some way it could be trans related? It has never popped out of gear.
I have an 03 KX250 and am thinking about putting the entire clutch assembly (from the basket to the springs) into the 98 and vise versa and changing one part at a time until it starts slipping. That's if the clutch assemblies interchange. Plus it's gonna take a long time to test each part 1 by 1.
If anyone has any advise I'd appreciate it. Thanks.

flynbryan - It slipped with the White Bros. and OEM 250 springs so I figured I would give the OEM 500 springs a shot. I also took the complete clutch assembly to a MX motor/suspension shop (when I was having some fork seals replaced) and he told me everything looked ok. I tried excessive free play in the lever and it still slipped. Everything in the clutch assembly is to spec. Your right about the Hinson basket, even after talking to them it didn't seem like a basket problem to me. Thanks for the help.
Ray30+Vet - After I put the 2nd new clutch in I figured it had to be in the trans. But I've had my son's KX60 trans apart and couldn't see how that could cause the problem. But know you got me thinking. I should probably split the cases and have a look around. The bike is just sitting there and it's not gonna fix itself. But 1st I'll see if the clutch assemblies interchange between 98 and 03. If it slips with my 03 assembly it HAS to be in the trans.
Right?
If anyone else has an opinion I'm all ears.

john stu - I have the service manual. I have went over the clutch assembly with a fine tooth comb at least 10 times. It's been a major bummer.
Ray - Thanks for the help. It is kinda refreshing knowing someone else has had a similar problem. Hopefully I'll be able to work on it next week and I'll post what I find.