The 98 mach1's are well known for many problems like this. I have the same sled, the main cause for electrical problems on this sled is the main wiring harness. It runs under the engine and was made to short, it rubs on the bottom of the engine or the mounts under there. I have not had this problem yet, but from what i've read, the only fix is to pull the engine out and check the wiring harness, and while you have it out extend all the wires a little. Check out this other web site: dootalk.com
There is a whole forum just of the mach sleds, lots of helpful info on there.
If you go there look for the machz forum, under that is a subforum called machz/mach1 triples, thats the one you want. I've gotten alot of help from there.