I have a 99 (S reg) impreza sport 2.0 AWD, manual gbx. i have recently started having trouble shifting into 1st, from 1st to 2nd and back down to 2nd from 3rd. the only thing that sometimes solves this is going to neutral, jiggling the gear stick around in neutral and trying again. not fun at roundabouts! also have had trouble for a while with it slipping out of 5th if i'm not applying the accelerator. please help

there is a vacuum hose that runs from the inlet manifold down to the gear actuator on the transmission
that unit uses the manifold ( throttle setting) vacuum to change the ratios appropriate to engine load
the hose is prone to cracking , breaking or falling off

Sounds like the shift lever is not coming completely out of 2nd gear. So, by shifting into 1st gear, you recenter the shift lever. You can confirm this by going under the truck and moving the 1st - 2nd lever on the side of the transmission with your hand. If the shifter goes into reverse after you manually recenter the 1-2 lever, then you need to correct the problem that prevents the "normal" recentering of the 1-2 shift lever.

Possibilities are: Bent shift rod; worn shifter; worn shift rod at connection to shift lever on transmission.

if it has a kick down cable that could be the problem look for a cable running from throttle to trans that can be adjusted in length

Your transmission is in a "limp in" mode. There are 2 sensors that need replaced. Total cost should be less than $100. I had it happen to me on my 99 300m. The sensors limit the speed to about 25 mph and the RPM rev's high. Just get the sensors replaced.

YOU NEED TO TAKE YOUR TRANS OUT AND HAVE IT REBUILT, THE SYNCHROS ARE WORN OUT!!
THAT IS WHY YOU HAVE TROUBLE SHIFTING GEARS!!

transmission?

Have you tried taking the car in for maintenance? Usually on most (older) auto transmissions there is a hose coming off of the exhaust manifold or carburetor area that provides the pressure for the transmission fluid. If that hose doesn't have a good seal, your shifting may be difficult and the transmission may not stay in the gear you want it to.

If you can see where the shifter goes through the floorboard, make sure the shifter is not hitting the floor.
There could be interfierence under the car also.