94 LHS runs suddenly bad while cruising

I have had this 94 lhs for about 5 yrs.had 89.000 MLS when I bought it now it has 208.000. Regular oil changes and upkeep timming belt changed couple of years ago and new wires and plugs couple years ago as well.

For the last year the car would just suddenly start running real bad and missing when driving along at cruise speed. I thought that it was a fuel pump problem may still be...so I would fill it up with premium gas and it would run ok then when the tank would get low it would run real bad again. So I did this all the time untill about two weeks ago when it just started blowing smoke and running horribly on a full tank so I had it towed to my repair guy. He checked the car and determined a blown head gaket.

So I had him replace both head gaskets and new wires and plugs as well as he replaced serpentine belt (?) $1,100 bucks later he ran the car 4 miles square and I picked it up still 3/4 of a full tank of gas. The car ran real good but I still sensed some slite missing at idle say at a stop sign in gear. Then last night night at a quarter tank the car did the same thing it had always done in the past year and started missing and running real bad ...I pulled off the road,Wanting to beet the car with a baseball bat,shut the car off started it again and it ran good all the way home about 10 miles.

I was going up a hill with a slite turn to the right when it started running bad. Any Idea what the heck is going on. Is this a fuel pump type problem. he also cahnged the fuel filter.
Thanks
Mr six

It may be a bad 02 sensor, have the machanic check it.
Good luck.