trans issues

I have a 95 Chevy 3/4 ton with a 350 and a 5 speed manual trans its 4x4 and has about 155,000 on it. I am having some issues with it shifting once it gets warm. Once it gets to a good operating temp and its been through all the gears a few times the trans seems like it wants to keep me from shifting into a lower gear. In town this can be a big pain in the butt. But I have found that if I will pump the clutch a few times it will usually go in gear unless it wants to be a pain in the neck. I have about had it with this truck and am thinking of trading it in unless yall have some suggestions on what it might be. I took it to my local shop and all he told me was that he was gonna drop the transmission and whether I fix it or not hit me with a huge bill. I dont mind doing the work myself just got to know what the heck is going on with it and whats causing this. Some people have even told me that its a normal Chevy problem with standards and I should just get used to it. Y'alls help would be greatly appreciated.