Clutch/Transmission problem please help

I have a 1987 b2000 with 69,000 miles on it. Got it last November from a buddy, had a few problems, got them fixed for what I thought but found out today not totally fixed...the trucks not my daily driver, use mainly for winter time. When i first got the truck it had been sitting for a few months. On to the problem, it shifted into all gears fine when not running, once running it wouldn't go into 1-5 with clutch fully pressed in. I didn't want to force it into gear to hard, I put it into reverse and it grinded a little but but was able to get into gear and back up a hill, clutch still wasn't working pressed in it was like i was in full reverse. I forced back into neutral and went for first still nothing, I started rolling down the hill and gears synced up and all of the sudden the clutch started working as normal shifting with no problem at all. Drove it all winter as my daily driver no problems at all. Today I went to start it up as it has been sitting for about a month or so to have the same problem with the transmission lockout once started, but once rolling and the gears synced up the clutch works just fine and shifting just as smooth as before drove around throughout the day seeing if once i started it back up it would reoccur but have had no problems since. Any help is greatly appreciated, tried my best to explain let me know if you need more clarification. Thanks again

I think I'd replace both the clutch master and clutch slave cylinder on it if it was mine. Your miles are low, but the years ain't. So in this case, opposite of what Indiana Jones said ("it's not the years, it's the mileage").

That's the plan if I can't figure it out, just really weird that it works fine unless it sits for awhile.