Bad grounds can make modern cars do weird things. To eliminate this possibility remove all ground straps, clean to bare metal and re-install. Do the same with battery connections.

With that many miles the rings/valves are probably worn - heavier wt oil can help with that. If you recently changed the oil, maybe try adding a pint of STP or other viscosity-booster. In summer, the added heat will exacerbate this issue.

A compression check will tell you how worn the engine is. If the comp is low, first step would be to pull the head and do a valve job. Second step would be to find a low-mileage engine from a wreck....