Bertis, definitely check the Gear position selector (might also be called a neutral lockout switch or similar). On the 97LE (which isn't what you've got, but should be similar (97LE is V8 EF/L), its a cream plastic box that sits on the side of the gearbox (has a shield over it, sits on passenger side). You can pull it off and check the resistance values with a multimeter, and this will give you a reasonable indication of whether its working or not.
Basically, its used to tell the ecu where you have the selector, and uses resistance values to do this. I've had this problem previously - gradually the switch dies and sends erratic values to the ecu, which will freak out.
Mine occurred randomly for about a 2 weeks, then got very bad on a ski trip
Similar symptoms to you, it would randomly pick another gear, (including neutral), and wouldn't respond to the selector in terms of gears. Made highway driving exciting to say the least (2nd at 100 is fun isn't it ?
For the 97LE, it was $85 for the new switch, and took all of 15 mins to install.
If it is the switch, and you are planning conversion very soon, you could probably get a second hand switch from the wreckers - but imo this is a common problem, so the switch you get from the wreckers might not last long . . .