Odd acceleration issue

[Mgicm]

Having an odd issue. Have a 72 stepvan with 250v6. Drives ok around town. Get on highway...wont go above like 45mph. After about 3-4 minutes...it starts to go faster to like 50-55mph. RUns ok speed after that. If i get off highway... turn around and get back onto highway same thing. Slow at like 45mph and then out of nowhere goes to 55+ for no apparent reason. No hills or anything .

Sometimes from a stop it does seem to take some time to get up to speed.

Could this be an issue related to ignition timing etc?

thanks!