jumping out of 5th gear

[darkt]

Hi , been a while since I've been here, cars been running quite well for a good amount of time but time for a question, 

My car is a 2004 2ltr petrol with just short of 100 thousand miles and now car keeps jumping out of 5th gear, can keep it in gear if 

I hold gear stick in but as soon as pressure is realised just jumps out into neutral.

So is there anything on this site to help me with instructions for replacing 5th gear, been told it's an easy job but would love to get some advice

before I end up with a bag of bits. Parts from Toyota are only £168 but some of the prices I've been quoted from garages have gone as far as £700 to do job.

Any help would be appreciated, got most tools I need but do not want to be sat there , no car and find I need a special tool that takes a fortnight to arrive.

[furtula]

I presume you already tried replacing the oil, or readjusting the shift linkage?