G'day Bill and welcome to the forum.
I'd take a peek inside the turbo and see how it looks. It's possible that it's burnt some oil during shutdown and started a decline. Might be an idea to get it seen to before it lets go - I don't think your model had an intercooler to catch the broken bits, which means it could toast your engine.
Do you let the engine idle for a little while after you've run it hard, or do you drive it gently for a while after getting off the freeway so the turbo can spin down and cool down?
A hot, spinning turbo needs lubrication and if the engine stops, the lube stops. Many people fit turbo timers to alleviate this - maybe that's something you could consider as well. Make sure it's one that monitors EGT.