I'm servicing a commercial customer with some Juno 4" cans in a display case and they keep cycleing off and on. The lights are mounted above the merchandise in a seperate compartment that seems fairly large. There's plenty of room for the non IC cans and their base plate and junction box. There is no thermal switch mounted right on the can but some sort of a black cylindrical set of contacts sticking out of the junction box. This device immediatly gets hot when the lights are turned on as if the current of the lamp is overheating it. I'm using 50PAR20 lamps although the fixture says 50R20 or 50PAR16. The problem just started after years of service using the 50PAR20 lamps. Are those contacts replacable? Is there a 30PAR20 lamp?

Yes, you can replace the thermal....BUT, it must be JUNO's.
Yes, there is a 30PAR20 lamp (WW Grainger Catalog)
The fixtures are 'cycling' due to the thermal OL; be it old age, wrong lamps for to long, or poor ventilation, or a combo of the above reasons.
Also, bulb (30) is physically larger than 20, and PAR bulbs reflect heat back, as opposed to "R" lamps. The PAR heat is the 'root' of the problem
John

Would it be a thermal overload even though it's mounted in the j-box, 8 inches away from the lamp?

There are/were some thermals that functioned via the current draw. I do not know if Juno used/uses those.
John