My boxster has the same symptoms as yours but mine has 114,000 miles and counting. I haven't done that 3rd gear test but i feel my clutch still has some life to it and it doesn't slip except for that gitter like motion starting to move from first gear as you described it. And i don't have the IMS gaurd. My leak was located on the passenger side of the engine which led me to belive that one of the spark plug tubes was no good.
I already did the major service with new tubes and will look at the engine when i take off the wheels sometime soon to see if my theory was right. But i do get that burnt rubber smell. also i checked my filter and it was super clean so that means all the metal has completely shaven or the ims is still good.
In my case ill wait for the clutch to go and then do all those changes along with the ims. But I don't worry too much because if something were to go wrong well I would part my baby and use that money for another boxster. In the end i think that would be a cheaper solution then doing and engine rebuild or the ims change too early. But then again we do grow attached to our cars and it is always sad to see something go wrong that we could have avoided.
So in your case your car is still quite young and the best thing to do is what will help you feel safer. If you feel that not changing the ims will cause you to have horrible nightmare about a dead boxster then change it, otherwise wait for the clutch as thstone said. That is the best advice I can think of. Good luck.
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