irratic power surges idling or driving
 

I have a 98 gmc 6.5 turbo 3500 automatic. I am worse then being new to diesels. I just bought this truck and no NOTHING about diesel engines. I have a half decent back ground in gas engines. this is my problem. the truck seems to run fine (not sure what to expect from a diesel) it idles smooth and drives smooth for the most part. I get irratic power surges during driving. I will be drive say a steady 35 and the truck will all of the sudden get a surge of power that will can last for a half a second or up to as longs as 30 seconds or longer. most of the time I have to let off the pedal due to major increase in speed. when i let off the pedal it goes back to normal for a while. I am thinking that maybe this surge of power that I am getting is the way the engine is suppose to run. but I really dont know. I have no engine lights, no stalling, no backfiring or cutting out, no hesitation. I can hear what I assume is a whistle from the turbo. I change the fuel filter, cleaned the air filter, put in a fuel additive with no change. I am changing the oil today. I really dont know where else to look due to the lack of knowledge. could some body pleeeese get me some pointers in things to look at. I dont have any after market gauges, (boost, fuel, ect..) no mods with the exception of the pmd. was mounted on the top of the intake (which I will be relocating soon.) all the info that it looks like I know is only what I have learned in the last few days (thank god for google) would love to get some pointers from someone that knows more then me (which is probably anyone on this site.)

Sounds like a fuel issue maybe the LP (lift pump) is going bad and relocate the PMD today out of the engine compartment dont wait for it to go bad

I checked the lift pump. by starting it and opening the water purge valve. the engine stayed running. Also I could here the lift pump running so I am assuming that it is working. I after doing some more research I am starting to think I have a blown head gasket. I took it up the mountain yesterday pulling a small trailer. it couldnt get out of its own way. I brought it back home and started looking for other things. I popped the cap off of the radiator fill bottle and woosh!! she is spewing all over the place. I am now thinking I have a blown head gasket!! is there anything that would cause lots of pressure in the cooling system on startup other then a blown head gasket? if so is there anyway to run an easy test with basic tools. last question. what is the average cost of head gasket replacement at a garage? should I just do it myself. as I said in my 1st post, I have experience in engine work just not diesel.

Check top radiator hose on cold start up rock hard = head problem or gasket do by all mean get the heads checked before reassembly as 6.5s a prone to cracking around cylinder 8