electrical problems?

I just found this site and really need help. I have posted this on many other forums and have not had any luck. I have a 92 silverado with a 5.7/700r combo and 2 wheel drive. The problem ocurred about a month ago and is getting worse. First it started when I would use the heat/ac blower on medium or high the truck would die as if it had no spark. I have disconnected everthing related to the heat/ac system with no luck. Recently the speedo has been going nuts but all other guages are fine. I have ripped apart the dash multiple times hoping to find something possibly a bad ground and haven't come up with anything. Since the speedo thing has started the truck misses when the speedo is not working correctly but when working correctly the truck runs strong. I have no codes and have just been unplugging sensors and wires all over to find the problem. I beleive it is a ground problem but can't seem to find it. I am not sure how the scan tools work but have been thinking about investing in 1 but not sure it would point me to the problem if it was a wire. Any help is greatly appreciated.

Made a little progress. I found on another forum where if you jump a and c on the igniton module you bypass the ecm advance timimg. Well I figure unhooking the est wire does the same so I did this. The truck will run with the heat/ac on high and the speedo stays at 0 when idling. When I hook the est wire back up the truck will not run with the heat on and the speedo goes nuts again. So tommorrow I will try a new module but if that is not it then it is the ecm correct?

sounds like your on the right track

PROBLEM SOLVED! It was the Ecm ground. Had a bad spot in the wire and I couldn't locate it