My 97 plymouth neon is misfiring?

i had purchased the vehicle about 3 weeks ago and had the water pump replaced 2 days after. drove fine for 2 weeks until the snow storm (i'm a contracted courier and deliver in my own vehicle) i was in somerset ma when it started to jerk or misfire while driving in the slush and rain. it worked and then left me stranded like 3 times. i had it towed to my mechanic and they couldn't find out whats wrong with it without taking the engine apart (i couldn't afford that) they replaced the throttle positioning sensor and got it to idle normally (after many attempts to star it) but if given gas it either bogs out or misfires making the engine rev for 1 second intervals... any idea on what this can be? plugs are new, wires are new, alternator is fine.... it was missing the map sensor when purchased but i highly doubt that's the problem.