Power Problem

Hello, any help in diagnosing this problem would be much appreciated. I have a 1985 GMC Jimmy Full-size with a 305 V8, VIN H, carbureted with a 4BBL Edelbrock carb. The guy I bought it from removed all of the emissions equipment from it and also instaled some valve covers without a PCV Valve. I've always noticed that it lacked top end power when driving it but more recently I had a new problem arise. Today I passed someone while driving then shortly after my truck started to gun forward then draw back, the engine was gunning then falling back, like it was being governed. It's been doing it all the time now under a heavy load (when climbing hills) or when on a straight-away. It seems to do it in third gear. I've also noticed that I have bubbles in my water when the truck is running, as noted in the extra water tank for the cooling system. I think I have a blown head gasket but am not certain, going to have a compression check done on all of my cylinders soon, but also could I be having a problem with my transmission causing this? The gunning forward and dropping back. I've also noted that it has a clanking noise when shifting from first to second gear and vice versa. Lots of work to come...any help appreciated.

- Matthew