Just a guess. The power steering pump is going out or, the belt is slipping. Possible, the rack assembley needs replacing. The 88 could have either the Z or Cam rack assembly. Check the boots. One unit uses greese while the other uses oil. See if they have fluid. Suspension parts can be binding. IE..Tie rod ends, Ball joint, Upper bearings in strut mount. You said it was increasingly more difficult, that, and the fact it started in the cold, is typical of the rack.. Not really a bad job. Two U-bolts, fluid lines, tie rod end removal and steering joint.
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Max..1989 244 DL 5 Spd., V15 Phase II Cam Bilstein HD, Turbo Swaybars, Poly Bushings all round, Turbo Wheels, Black leather interior, Electric mirrors, LED dash and gauge lights and now NEW ECODES with the turn signals, 1992 black 244 next project