I'm new to the whole car mechanics thing...but am a fast learner...anyways the whole history is got it about a year and a half ago, the msd distributore that was in it went bad, had it replaced with an hei distributor and the wiring fixed bcause he partly hacked it up to put it in. Ran great, past two weeks been getting up to about 25-35 not wanting to shift into higher gears, will still run but once it gets to a certain mph is stays there and revs...replaced vacuum modulator...checked fluids in tranny (they were fine)...now it shifts hard when it does shift, and died on me this morning going up the hill to my house...where it pretty much bogs down to about 20 mph...help!

 

forgot to mention the engine and tranny were rebuilt before I got it...some minor modifications were done...like fixing the exhaust (which ended before the drivers door) and replacing the donuts

 

what trans?

also is there vacuum going to that modulator?

i had a similar issue in a old th350. it did shift normal but when it shifted the truck would slam on the brakes basically. ended up being a bad sprag inside the transmission.