I have to say I have no expert knowledge of the 300TE. However....
On any automatic car my first port of call would be the auto transmission for the symptoms you describe.
Is the fluid at the correct level? Low fluid levels could cause this problem.
When was the last time you had an oil and filter change in the transmission?
Does the auto oil look a nice clear red colour or does it look burnt?
If all these items check out ok your probably looking a replacment transmission, I'd get it checked by a specialist you trust.
Good luck.
Alan M.