First, does the engine turn when cold?
What kind of engine oil are you using?
There are two parts that can cause slippage on the 2005. The ordinary Z-lock gripper thingy behind the flywheel (cannot remember the proper name for it), and there is a safety clutch next to the starter motor (under the round lid next to the clutch) which will save the starter if the engine backfires.
If you have problems with the second one, the starter turns, but the engine won't, it just makes a horrible squealing noise. It is possible to add a shim to this, increasing the forces on the "clutch plates". Be careful when removing the gear, it has a shim at the inner end of it's axle, and it can fall down into the engine whe the gear is removed. To fix this you need to press the wave shim down to get the clip out, then add a 0.5mm shim and reverse the process.
A worn z-lock gripper in combination with high viscosity oil is most likely.
I'll try to find the proper name for it, it is available directly from manufacturer, wich saves you a bit of cash compared to buying it as an original spare part.
