I would look elsewhere besides the PC itself as the problem.
! There is NO reason to change to another platform (such as Mac) as there is no benefit to doing so !
Macs etc. do NOT have better timing than PCs.
You mention only having an MPU and three synths, are you daisy-chaining them (IN-THRU etc.)? If so, that's probably your problem.
Pick up a good multi-port MIDI unit.

Up until I purchased my PIII-700 I was using a P-200 with a MOTU to manage 9 synths and effects, and I have no timing problems at all.

David
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