gearbox problems, can't shift into 4th and 5th gear Options

[Hydra]

Ok, here's what happenned: i drove my teener around last night and everything seemend to go great, all gear engaged very smoothly (except for 1st of course), then this morning when i took it for quick dash around the block i tried to go up to 4th gear but the lever just wouldn't go far enough to the right, i would always end up engaging 2nd gear... i drove the car back home and did a quick check of the bushings and linkages, everything seems to be fine. tried going into 4th again with the car turned off, but still nothing; the lever just refuses to go far enough to the right, then all of sudden it works again, but since it's always too good to be true, it only lasts a couple of shifts.
now i'm worried it's got something to do with the tranny's internals.
any feedback on what could be happenning would be great, since i've got a whole driving weekend planned.
thanks guys
Nick