An easy, free way to check another possibility is the waterpump weep hole to see if it's damp/leaking. I had a problem where the car would run fine until it started warming up. Come to find out when the coolant system would build pressure it would leak onto the optispark getting it wet and the car would stumble under load.
I just let the opti sit for a few days, changed the WP and it was good to go.