ds,
I read the sonnax info. sheet and your further description of soft shifts/slippage leads me to agree that this may be the problem.
I use a lot of Sonnax parts...great problem solvers.
I would recommend you try replacing the spring before you burn out the tranny. Spring seems to be replaceable without removing the valve body, and if
VB removal is needed, it's not that bad...just messy laying on your back with fluid dripping on you.
You should also have a set of
VB gaskets on hand and an ATSG manual to help keep track of any check balls, etc. , just in case the
VB needs to come off. If you can do the fluid, you should be able to do this. If you need parts source info., let me know, I can get it or link you up.