Kia Sportage 2000

This has been a difficult issue for me for the last year or so. I have a manual transmission 2000 Kia Sportage and the problem I have been having with it is just getting progressively worse. Sometimes, always in hotter weather, when I have been idoling for a while (usually in traffic), and after driving my car for at least 15 minutes, it won’t go into gear. I can switch it into 1st gear or reverse out of neutral, but when I press the gas it just revs up (just like pressing the gas while in neutral). It leaves me dead stuck in the middle of the road and I usually have to get it towed or leave it aside for a while (till night) and come back and get it later. Then it just starts up and goes into 1st or reverse no problem. I have had the clutch replaced and the whole gear shift replaced. It is still doing to same thing and progressively getting worse, as it happens more frequently and now on short distance drives. The few mechanics I’ve taken it to are stumped! Is there anyone out there that can help me pin point the problem?