Minuous, that processor seems a little borderline for the newer WinUAE versions, but JIT should still give you better performance than normal modes.
How much RAM do you have in your PC? You'll need memory for the Amiga AND 4 MB for the JIT cache. You may have been allocating too much memory for your system to handle or not giving JIT a cache at all.
Give me your PC specs and the following:
Chipset you use (AGA, ECS, OCS) in Workbench?
Amiga RAM normally used in WinUAE (chip + fast) ?
.....and I'll place a copy of a JIT enabled WinUAE 0.8.17R3 in the zone setup and ready to go.