Help starter driving me nuts!....

[Lippyp]

Help! I had to replace my starter motor as my old one died on me after too much heat soak, the first remanufactured unit didn't work at all, the second one I have on now starts fine when cold, but after driving for a short while spins over fine but doesn't engage with the flexplate and so doesn't spin the engine and makes a horrible noise, do I just need to add another shim or two under it or is this starter faulty as well? I can see marks on the flat front edge of the flexplate where the starter drive seems to be hitting the flexplate and not engaging with the teeth. This is my first small block ever and I don't really know what I'm doing with it as all the starters I've worked on before bolted straight into the bell housing of whichever engie it was rather than to the block and needed no adjustment. I've checked with the little rod they supply with the motor and to my untutored eye it seems fine. I just don't understand why it's fine when cold and will start over and over but won't work when hot. Oh and I also don't currently have a brace on the back of the starter as there wasn't one on there when I bought the truck and chevy's just don't exist in junkyards over here.

Please help me before I curl up in a corner and cry with frustration! I've got a heap of garden and construction rubbish getting bigger and bigger in the garden and my wife is giving me meaningful looks when she walks past it so I need to get my truck working to get it to the dump or I may never have sex again!