Whilst in rapid rate of climb, I noticed the ASI decreasing and altimeter beginning to freeze. Normal level flight attitude and cruise power settings were selected and the ac flown at normal AoA. ASI and altimeter continued to give faulty indications.
This was in a F4. I called my leader and suggested dumping fuel and recovering in formation. However, being a press-on-itis flight commander, he wanted the exercise completed first...... Eventually we recovered in close formation - but he insisted on staying in cloud even though we could see the ground quite clearly through large holes and knew where we were. We flew the GCA, then the fool said that we'd overshoot into a visual circuit 'for practice'. I'd had about 20 minutes in close formation at this stage and was pretty pi$$ed off - but even more so when he declared his intention to overshoot from the visual circuit for another! The instruments still weren't working properly and the altimeter was hunting in synchronisation with the normal cabin pressure surges, although it seemed to settle when I pulled the vent knob. At this stage my navigator said to him "This is just practice bleeding - we've got actual instrument failure, so let's stop dicking around!" So we landed; in the debrief we told him what we thought about his poor leadership in delaying a safe recovery......
It turned out that a static pipe to the navigator's altimeter had become disconnected and cockpit air pressure was getting into the static system as a result!
The navigator left the RAF a few years later to become a senior bod in an organisation providing military support flying, I was posted to the VC10 about a year later......but, of course, the flight commander was promoted.