memory speed

Had my p4 1.8a running at 2.4g on my asus p4t
board and because of high temp's on the cpu changed out the hsf to a better one and also re-did the v-pin mod from 1.8 to 1.85. the motherboard undervolts anyway. now running 1.78vcore.after the change can't boot at 133 anymore! had to shift down to 125 because of lock-up and computer shutdowns! Am running samsung
orginal rdram 16 device and havent had any problems till this! cpu running at 39c idle and
up to 50c stressed. running 42c will surfering
internet. Did the memory go south! running okay at 125fsb. need help. really wanted to push it,but went the other way.

I doubt it's the memory.
You changed the hsf and now it wont boot at 133 like it did before you changed the hsf so I would look into something related to that first.