Your description is like a worn out clutch (standard shift) however it should have completly failed by now if a normal wear out. If an automatic: check the fluid level first. If way low (warmed up engine) you need to find the leak.
If you go to a transmission shop with it working properly it's going to be guessing. Fluid color could be a good clue - if dark it's not a good sign. It's going to tough to figure this out if it has worked OK since last May. You might have some luck with a good trans shop if they've seen this before - I haven't seen too many intermittent problems with a transmission but theres always a first :-(. Doesn't leave you very confident in this car I'm sure, good luck and as we all should, bring water, blankets, a cell phone.
Let us know if it's an automatic or not - perhaps we can take care it better. - T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/