Mystery... SSD Unstable inside MBP, stable perfect outside MBP???

Anyone have any ideas about this?

Late 08 unibody MacBook Pro.

Tried numerous SSD's (see other threads) and now on my 3rd.

The Crucial C300 works like a dream outside in a usb enclosure booting from that, great speed all you would expect etc etc.

Take it out and put it inside the default HD area and problems appear, lots of spinning beach balls for very small tasks, sometimes better than others always there though..

My existing default HDD works fine inside and out outside.

Is there anything i can do or will it just never work properly inside?

I've tried all the usual SSD ideas